diff --git a/.luacheckrc b/.luacheckrc
index 670c84325..2fe7e6915 100644
--- a/.luacheckrc
+++ b/.luacheckrc
@@ -22,7 +22,15 @@ read_globals = {
 	"ValueNoise", "ValueNoiseMap",
 
 	string = {fields = {"split", "trim"}},
-	table  = {fields = {"copy", "copy_with_metatables", "getn", "indexof", "keyof", "insert_all"}},
+	table  = {fields = {
+		"copy",
+		"copy_with_metatables",
+		"getn",
+		"indexof",
+		"keyof",
+		"insert_all",
+		"merge",
+	}},
 	math   = {fields = {"hypot", "round"}},
 }
 
diff --git a/builtin/common/misc_helpers.lua b/builtin/common/misc_helpers.lua
index 29aa3e5c2..717732d53 100644
--- a/builtin/common/misc_helpers.lua
+++ b/builtin/common/misc_helpers.lua
@@ -579,6 +579,15 @@ function table.insert_all(t, other)
 end
 
 
+function table.merge(...)
+	local new = {}
+	for _, t in ipairs{...} do
+		table.insert_all(new, t)
+	end
+	return new
+end
+
+
 function table.key_value_swap(t)
 	local ti = {}
 	for k,v in pairs(t) do
@@ -872,3 +881,47 @@ function core.parse_coordinates(x, y, z, relative_to)
 	local rz = core.parse_relative_number(z, relative_to.z)
 	return rx and ry and rz and { x = rx, y = ry, z = rz }
 end
+
+local function class_try_return(obj, ...)
+	if select("#", ...) ~= 0 then
+		return ...
+	end
+	return obj
+end
+
+local function class_call(class, ...)
+	local obj = setmetatable({}, class)
+
+	if obj.new then
+		return class_try_return(obj, obj:new(...))
+	end
+
+	return obj
+end
+
+function core.class(super)
+	local class = setmetatable({}, {__call = class_call, __index = super})
+	class.__index = class
+
+	return class
+end
+
+function core.super(class)
+	local meta = getmetatable(class)
+	return meta and meta.__index
+end
+
+function core.is_subclass(class, super)
+	while class ~= nil do
+		if class == super then
+			return true
+		end
+		class = core.super(class)
+	end
+
+	return false
+end
+
+function core.is_instance(obj, class)
+	return type(obj) == "table" and core.is_subclass(getmetatable(obj), class)
+end
diff --git a/builtin/game/init.lua b/builtin/game/init.lua
index b3c64e729..292975678 100644
--- a/builtin/game/init.lua
+++ b/builtin/game/init.lua
@@ -2,6 +2,7 @@
 local scriptpath = core.get_builtin_path()
 local commonpath = scriptpath .. "common" .. DIR_DELIM
 local gamepath   = scriptpath .. "game".. DIR_DELIM
+local uipath     = scriptpath .. "ui" .. DIR_DELIM
 
 -- Shared between builtin files, but
 -- not exposed to outer context
@@ -39,6 +40,7 @@ dofile(gamepath .. "hud.lua")
 dofile(gamepath .. "knockback.lua")
 dofile(gamepath .. "async.lua")
 dofile(gamepath .. "death_screen.lua")
+dofile(uipath .. "init.lua")
 
 core.after(0, builtin_shared.cache_content_ids)
 
diff --git a/builtin/ui/context.lua b/builtin/ui/context.lua
new file mode 100644
index 000000000..58137e129
--- /dev/null
+++ b/builtin/ui/context.lua
@@ -0,0 +1,143 @@
+-- Luanti
+-- SPDX-License-Identifier: LGPL-2.1-or-later
+-- Copyright (C) 2025 v-rob, Vincent Robinson <robinsonvincent89@gmail.com>
+
+ui.Context = core.class()
+
+local open_contexts = {}
+
+local OPEN_WINDOW   = 0x00
+local REOPEN_WINDOW = 0x01
+local UPDATE_WINDOW = 0x02
+local CLOSE_WINDOW  = 0x03
+
+function ui.Context:new(builder, player, state)
+	self._builder = ui._req(builder, "function")
+	self._player = ui._req(player, "string")
+	self._state = ui._opt(state, "table", {})
+
+	self._id = nil
+	self._window = nil
+end
+
+function ui.Context:open(param)
+	if self:is_open() then
+		return self
+	end
+
+	self:_open_window()
+	self:_build_window(ui._opt(param, "table", {}))
+
+	local data = ui._encode("BL Z", OPEN_WINDOW, self._id,
+			self._window:_encode(self._player, true))
+
+	core.send_ui_message(self._player, data)
+	return self
+end
+
+function ui.Context:reopen(param)
+	if not self:is_open() then
+		return self
+	end
+
+	local close_id = self:_close_window()
+	self:_open_window()
+	self:_build_window(ui._opt(param, "table", {}))
+
+	local data = ui._encode("BLL Z", REOPEN_WINDOW, self._id, close_id,
+			self._window:_encode(self._player, true))
+
+	core.send_ui_message(self._player, data)
+	return self
+end
+
+function ui.Context:update(param)
+	if not self:is_open() then
+		return self
+	end
+
+	self:_build_window(ui._opt(param, "table", {}))
+
+	local data = ui._encode("BL Z", UPDATE_WINDOW, self._id,
+			self._window:_encode(self._player, false))
+
+	core.send_ui_message(self._player, data)
+	return self
+end
+
+function ui.Context:close()
+	if not self:is_open() then
+		return self
+	end
+
+	local close_id = self:_close_window()
+	local data = ui._encode("BL", CLOSE_WINDOW, close_id)
+
+	core.send_ui_message(self._player, data)
+	return self
+end
+
+function ui.Context:is_open()
+	return self._id ~= nil
+end
+
+function ui.Context:get_builder()
+	return self._builder
+end
+
+function ui.Context:get_player()
+	return self._player
+end
+
+function ui.Context:get_state()
+	return self._state
+end
+
+function ui.Context:set_state(state)
+	self._state = ui._req(state, "table")
+	return self
+end
+
+local last_id = 0
+
+function ui.Context:_open_window()
+	self._id = last_id
+	last_id = last_id + 1
+
+	open_contexts[self._id] = self
+end
+
+function ui.Context:_build_window(param)
+	self._window = self._builder(self, self._player, self._state, param)
+
+	ui._req(self._window, ui.Window)
+	assert(not self._window._context, "Window object has already been used")
+
+	self._window._context = self
+end
+
+function ui.Context:_close_window()
+	local close_id = self._id
+
+	self._id = nil
+	self._window = nil
+
+	open_contexts[close_id] = nil
+	return close_id
+end
+
+function ui.get_open_contexts()
+	local contexts = {}
+	for _, context in pairs(open_contexts) do
+		table.insert(contexts, context)
+	end
+	return contexts
+end
+
+core.register_on_leaveplayer(function(player)
+	for _, context in pairs(open_contexts) do
+		if context:get_player() == player:get_player_name() then
+			context:_close_window()
+		end
+	end
+end)
diff --git a/builtin/ui/elem.lua b/builtin/ui/elem.lua
new file mode 100644
index 000000000..9d0d6274c
--- /dev/null
+++ b/builtin/ui/elem.lua
@@ -0,0 +1,140 @@
+-- Luanti
+-- SPDX-License-Identifier: LGPL-2.1-or-later
+-- Copyright (C) 2023 v-rob, Vincent Robinson <robinsonvincent89@gmail.com>
+
+ui._elem_types = {}
+
+function ui._new_type(base, type, type_id, id_required)
+	local class = core.class(base)
+
+	class._type = type
+	class._type_id = type_id
+	class._id_required = id_required
+
+	ui._elem_types[type] = class
+
+	return class
+end
+
+function ui.derive_elem(base, type)
+	assert(core.is_subclass(base, ui.Elem))
+	ui._req(type, "id")
+
+	assert(not ui._elem_types[type],
+			"Derived element name already used: '" .. type .. "'")
+
+	return ui._new_type(base, type, base._type_id, base._id_required)
+end
+
+ui.Elem = ui._new_type(nil, "elem", 0x00, false)
+
+function ui.Elem:new(param)
+	local function make_elem(props)
+		self:_init(ui._req(props, "table"))
+		return self
+	end
+
+	if type(param) == "string" then
+		self._id = ui._req(param, "id")
+		return make_elem
+	end
+
+	self._id = ui.new_id()
+	return make_elem(param)
+end
+
+function ui.Elem:_init(props)
+	self._groups = {}
+	self._children = {}
+
+	self._props = ui._cascade_props(props.props or props, {})
+	self._styles = {}
+
+	-- Set by parent ui.Elem
+	self._parent = nil
+
+	-- Set by ui.Window
+	self._boxes = {main = true}
+	self._window = nil
+
+	if self._id_required then
+		assert(not ui._is_reserved_id(self._id),
+				"Element ID is required for '" .. self._type .. "'")
+	end
+
+	for _, group in ipairs(ui._opt_array(props.groups, "id", {})) do
+		self._groups[group] = true
+	end
+
+	for _, child in ipairs(ui._opt_array(props.children, ui.Elem, props)) do
+		if core.is_instance(child, ui.Elem) then
+			assert(child._parent == nil,
+					"Element '" .. child._id .. "' already has a parent")
+			assert(not core.is_instance(child, ui.Root),
+					"ui.Root may not be a child element")
+
+			child._parent = self
+			table.insert(self._children, child)
+		end
+	end
+
+	for _, style in ipairs(ui._opt_array(props.styles, ui.Style, props)) do
+		if core.is_instance(style, ui.Style) then
+			table.insert(self._styles, style)
+		end
+	end
+
+	for _, item in ipairs(props) do
+		assert(core.is_instance(item, ui.Elem) or core.is_instance(item, ui.Style))
+	end
+end
+
+function ui.Elem:_get_flat()
+	local elems = {self}
+	for _, child in ipairs(self._children) do
+		table.insert_all(elems, child:_get_flat())
+	end
+	return elems
+end
+
+function ui.Elem:_encode()
+	return ui._encode("Bz S", self._type_id, self._id, self:_encode_fields())
+end
+
+function ui.Elem:_encode_fields()
+	local fl = ui._make_flags()
+
+	if ui._shift_flag(fl, #self._children > 0) then
+		local child_ids = {}
+		for i, child in ipairs(self._children) do
+			child_ids[i] = child._id
+		end
+
+		ui._encode_flag(fl, "Z", ui._encode_array("z", child_ids))
+	end
+
+	self:_encode_box(fl, self._boxes.main)
+
+	return ui._encode_flags(fl)
+end
+
+function ui.Elem:_encode_box(fl, box)
+	-- Element encoding always happens after styles are computed and boxes are
+	-- populated with style indices. So, if this box has any styles applied to
+	-- it, encode the relevant states.
+	if not ui._shift_flag(fl, box.n > 0) then
+		return
+	end
+
+	local box_fl = ui._make_flags()
+
+	-- For each state, check if there is any styling. If there is, add it
+	-- to the box's flags.
+	for i = ui._STATE_NONE, ui._NUM_STATES - 1 do
+		if ui._shift_flag(box_fl, box[i] ~= ui._NO_STYLE) then
+			ui._encode_flag(box_fl, "I", box[i])
+		end
+	end
+
+	ui._encode_flag(fl, "s", ui._encode_flags(box_fl))
+end
diff --git a/builtin/ui/init.lua b/builtin/ui/init.lua
new file mode 100644
index 000000000..1f46d470b
--- /dev/null
+++ b/builtin/ui/init.lua
@@ -0,0 +1,18 @@
+-- Luanti
+-- SPDX-License-Identifier: LGPL-2.1-or-later
+-- Copyright (C) 2023 v-rob, Vincent Robinson <robinsonvincent89@gmail.com>
+
+ui = {}
+
+local UI_PATH = core.get_builtin_path() .. "ui" .. DIR_DELIM
+
+dofile(UI_PATH .. "util.lua")
+dofile(UI_PATH .. "selector.lua")
+dofile(UI_PATH .. "style.lua")
+dofile(UI_PATH .. "elem.lua")
+
+dofile(UI_PATH .. "static_elems.lua")
+
+dofile(UI_PATH .. "window.lua")
+dofile(UI_PATH .. "context.lua")
+dofile(UI_PATH .. "theme.lua")
diff --git a/builtin/ui/selector.lua b/builtin/ui/selector.lua
new file mode 100644
index 000000000..a1db3225d
--- /dev/null
+++ b/builtin/ui/selector.lua
@@ -0,0 +1,569 @@
+-- Luanti
+-- SPDX-License-Identifier: LGPL-2.1-or-later
+-- Copyright (C) 2023 v-rob, Vincent Robinson <robinsonvincent89@gmail.com>
+
+ui._STATE_NONE = 0
+ui._NUM_STATES = bit.lshift(1, 5)
+ui._NO_STYLE = -1
+
+local states_by_name = {
+	focused  = bit.lshift(1, 0),
+	selected = bit.lshift(1, 1),
+	hovered  = bit.lshift(1, 2),
+	pressed  = bit.lshift(1, 3),
+	disabled = bit.lshift(1, 4),
+}
+
+--[[
+Selector parsing functions return a function. When called with an element as
+the solitary parameter, this function will return a boolean, indicating whether
+the element is matched by the selector. If the boolean is true, a table of
+tables {box=..., states=...} is also returned. If false, this is nil.
+
+The keys of this table are unique hashes of the box, which serve to prevent
+duplicate box/state combos from being generated. The values contain all the
+combinations of boxes and states that the selector specifies. The box name may
+be nil if the selector specified no box, in which case it will default to
+"main" unless/until it is later intersected with a box selector. This list may
+also be empty, which means that contradictory boxes were specified and no box
+should be styled. The list will not contain duplicates.
+--]]
+
+-- By default, most selectors leave the box unspecified and don't select any
+-- particular state, leaving the state at zero.
+local function make_box(name, states)
+	return {name = name, states = states or ui._STATE_NONE}
+end
+
+-- Hash the box to a string that represents that combination of box and states
+-- uniquely to prevent duplicates in box tables.
+local function hash_box(box)
+	return (box.name or "") .. "$" .. tostring(box.states)
+end
+
+local function make_hashed(name, states)
+	local box = make_box(name, states)
+	return {[hash_box(box)] = box}
+end
+
+local function result(matches, name, states)
+	if matches then
+		return true, make_hashed(name, states)
+	end
+	return false, nil
+end
+
+ui._universal_sel = function()
+	return result(true)
+end
+
+local simple_preds = {}
+local func_preds = {}
+
+simple_preds["no_children"] = function(elem)
+	return result(#elem._children == 0)
+end
+
+simple_preds["first_child"] = function(elem)
+	return result(elem._parent == nil or elem._parent._children[1] == elem)
+end
+
+simple_preds["last_child"] = function(elem)
+	return result(elem._parent == nil or
+			elem._parent._children[#elem._parent._children] == elem)
+end
+
+simple_preds["only_child"] = function(elem)
+	return result(elem._parent == nil or #elem._parent._children == 1)
+end
+
+func_preds["<"] = function(str)
+	local sel = ui._parse_sel(str, true, false)
+
+	return function(elem)
+		return result(elem._parent and sel(elem._parent))
+	end
+end
+
+func_preds[">"] = function(str)
+	local sel = ui._parse_sel(str, true, false)
+
+	return function(elem)
+		for _, child in ipairs(elem._children) do
+			if sel(child) then
+				return result(true)
+			end
+		end
+		return result(false)
+	end
+end
+
+func_preds["<<"] = function(str)
+	local sel = ui._parse_sel(str, true, false)
+
+	return function(elem)
+		local ancestor = elem._parent
+
+		while ancestor ~= nil do
+			if sel(ancestor) then
+				return result(true)
+			end
+			ancestor = ancestor._parent
+		end
+
+		return result(false)
+	end
+end
+
+func_preds[">>"] = function(str)
+	local sel = ui._parse_sel(str, true, false)
+
+	return function(elem)
+		for _, descendant in ipairs(elem:_get_flat()) do
+			if descendant ~= elem and sel(descendant) then
+				return result(true)
+			end
+		end
+		return result(false)
+	end
+end
+
+func_preds["<>"] = function(str)
+	local sel = ui._parse_sel(str, true, false)
+
+	return function(elem)
+		if not elem._parent then
+			return result(false)
+		end
+
+		for _, sibling in ipairs(elem._parent._children) do
+			if sibling ~= elem and sel(sibling) then
+				return result(true)
+			end
+		end
+
+		return result(false)
+	end
+end
+
+func_preds["nth_child"] = function(str)
+	local index = tonumber(str)
+	assert(index, "Expected number for ?nth_child()")
+
+	return function(elem)
+		if not elem._parent then
+			return result(index == 1)
+		end
+		return result(elem._parent._children[index] == elem)
+	end
+end
+
+func_preds["nth_last_child"] = function(str)
+	local rindex = tonumber(str)
+	assert(rindex, "Expected number for ?nth_last_child()")
+
+	return function(elem)
+		if not elem._parent then
+			return result(rindex == 1)
+		end
+
+		local index = #elem._parent._children - rindex + 1
+		return result(elem._parent._children[index] == elem)
+	end
+end
+
+local function is_nth_match(elem, sel, index, dir)
+	if not elem._parent then
+		return index == 1 and sel(elem)
+	end
+
+	local first, last
+	if dir == 1 then
+		first = 1
+		last = #elem._parent._children
+	else
+		first = #elem._parent._children
+		last = 1
+	end
+
+	local count = 0
+	for i = first, last, dir do
+		local sibling = elem._parent._children[i]
+
+		if sel(sibling) then
+			count = count + 1
+		end
+
+		if count == index then
+			return sibling == elem
+		end
+	end
+
+	return false
+end
+
+func_preds["first_match"] = function(str)
+	local sel = ui._parse_sel(str, true, false)
+
+	return function(elem)
+		return is_nth_match(elem, sel, 1, 1)
+	end
+end
+
+func_preds["last_match"] = function(str)
+	local sel = ui._parse_sel(str, true, false)
+
+	return function(elem)
+		return is_nth_match(elem, sel, 1, -1)
+	end
+end
+
+func_preds["only_match"] = function(str)
+	local sel = ui._parse_sel(str, true, false)
+
+	return function(elem)
+		return is_nth_match(elem, sel, 1, 1) and is_nth_match(elem, sel, 1, -1)
+	end
+end
+
+func_preds["nth_match"] = function(str)
+	local sel, rest = ui._parse_sel(str, true, true)
+	local index = tonumber(rest)
+	assert(index, "Expected number after ';' for ?nth_match()")
+
+	return function(elem)
+		return is_nth_match(elem, sel, index, 1)
+	end
+end
+
+func_preds["nth_last_match"] = function(str)
+	local sel, rest = ui._parse_sel(str, true, true)
+	local rindex = tonumber(rest)
+	assert(rindex, "Expected number after ';' for ?nth_last_match()")
+
+	return function(elem)
+		return is_nth_match(elem, sel, rindex, -1)
+	end
+end
+
+local function parse_term(str, pred)
+	str = str:trim()
+	assert(str ~= "", "Expected selector term")
+
+	-- We need to test the first character to see what sort of term we're
+	-- dealing with, and then usually parse from the rest of the string.
+	local prefix = str:sub(1, 1)
+	local suffix = str:sub(2)
+
+	if prefix == "*" then
+		-- Universal terms match everything and have no extra stuff to parse.
+		return ui._universal_sel, suffix, nil
+
+	elseif prefix == "#" then
+		-- Most selectors are similar to the ID selector, in that characters
+		-- for the ID string are parsed, and all the characters directly
+		-- afterwards are returned as the rest of the string after the term.
+		local id, rest = suffix:match("^([" .. ui._ID_CHARS .. "]+)(.*)$")
+		assert(id, "Expected ID after '#'")
+
+		return function(elem)
+			return result(elem._id == id)
+		end, rest, nil
+
+	elseif prefix == "%" then
+		local group, rest = suffix:match("^([" .. ui._ID_CHARS .. "]+)(.*)$")
+		assert(group, "Expected group after '%'")
+
+		return function(elem)
+			return result(elem._groups[group] ~= nil)
+		end, rest, nil
+
+	elseif prefix == "@" then
+		-- It's possible to check if a box exists in a predicate, but that
+		-- leads to different behaviors inside and outside of predicates. For
+		-- instance, @main@thumb effectively matches nothing by returning an
+		-- empty table of boxes, but would return true for scrollbars if used
+		-- in a predicate. So, prevent box selectors in predicates entirely.
+		assert(not pred, "Box selectors are invalid in predicate selectors")
+
+		-- First, check if this can be parsed as a universal box selector.
+		local name = suffix:sub(1, 1)
+		local rest
+
+		if name == "*" then
+			rest = suffix:sub(2)
+
+			return function(elem)
+				-- If we want all boxes, iterate over the boxes in the element
+				-- and add each of them to the full list of boxes.
+				local boxes = {}
+
+				for name in pairs(elem._boxes) do
+					local box = make_box(name, ui._STATE_NONE)
+					boxes[hash_box(box)] = box
+				end
+
+				return true, boxes
+			end, rest, nil
+		end
+
+		-- Otherwise, parse it as a normal box selector instead.
+		name, rest = suffix:match("^([" .. ui._ID_CHARS .. "]+)(.*)$")
+		assert(name, "Expected box or '*' after '@'")
+
+		return function(elem)
+			-- If the box is in the element, return it. Otherwise, the
+			-- selector doesn't match.
+			if elem._boxes[name] then
+				return result(true, name, ui._STATE_NONE)
+			end
+			return result(false)
+		end, rest, nil
+
+	elseif prefix == "$" then
+		-- Unfortunately, we can't detect the state of boxes from the server,
+		-- so we can't use them in predicates.
+		assert(not pred, "State selectors are invalid in predicate selectors")
+
+		local name, rest = suffix:match("^([" .. ui._ID_CHARS .. "]+)(.*)$")
+		assert(name, "Expected state after '$'")
+
+		local state = states_by_name[name]
+		assert(state, "Invalid state: '" .. name .. "'")
+
+		return function(elem)
+			-- States unconditionally match every element. Specify the state
+			-- that this term indicates but leave the box undefined.
+			return result(true, nil, state)
+		end, rest, nil
+
+	elseif prefix == "/" then
+		local type, rest = suffix:match("^([" .. ui._ID_CHARS .. "]+)%/(.*)$")
+		assert(type, "Expected window type after '/'")
+
+		assert(ui._window_types[type], "Invalid window type: '" .. type .. "'")
+
+		return function(elem)
+			return result(elem._window._type == type)
+		end, rest, nil
+
+	elseif prefix == "," or prefix == ";" then
+		-- Return nil instead of a function and return the prefix character to
+		-- instruct ui._parse_sel() to union or end the selector accordingly.
+		return nil, suffix, prefix
+
+	elseif prefix == "(" then
+		-- Parse a matching set of parentheses, and recursively pass the
+		-- contents into ui._parse_sel().
+		local sub, rest = str:match("^(%b())(.*)$")
+		assert(sub, "Unmatched ')' for '('")
+
+		return ui._parse_sel(sub:sub(2, -2), pred, false), rest, nil
+
+	elseif prefix == "!" then
+		-- Parse a single predicate term (NOT an entire predicate selector) and
+		-- ensure that it's a valid selector term, not a comma or semicolon.
+		local term, rest, _ = parse_term(suffix, true)
+		assert(term, "Expected selector term after '!'")
+
+		return function(elem)
+			return result(not term(elem))
+		end, rest, nil
+
+	elseif prefix == "?" then
+		-- Predicates may have different syntax depending on the name of the
+		-- predicate, so just parse the name initially.
+		local name, after = suffix:match("^([" .. ui._ID_CHARS .. "%<%>]+)(.*)$")
+		assert(name, "Expected predicate after '?'")
+
+		-- If this is a simple predicate, return its predicate function without
+		-- doing any further parsing.
+		local func = simple_preds[name]
+		if func then
+			return func, after, nil
+		end
+
+		-- If this is a function predicate, we need to do more parsing.
+		func = func_preds[name]
+		if func then
+			-- Parse a matching pair of parentheses and get the trimmed
+			-- contents between them.
+			assert(after:sub(1, 1) == "(", "Expected '(' after '?" .. name .. "'")
+
+			local sub, rest = after:match("^(%b())(.*)$")
+			assert(sub, "Unmatched ')' for '?" .. name .. "('")
+
+			local contents = sub:sub(2, -2):trim()
+			return func(contents), rest, nil
+		end
+
+		-- Otherwise, there is no predicate by this name.
+		error("Invalid predicate: '?" .. name .. "'")
+
+	else
+		-- If we found no special character, it's either a type or it indicates
+		-- invalid characters in the selector string.
+		local type, rest = str:match("^([" .. ui._ID_CHARS .. "]+)(.*)$")
+		assert(type, "Unexpected character '" .. prefix .. "' in selector")
+
+		assert(ui._elem_types[type], "Invalid element type: '" .. type .. "'")
+
+		return function(elem)
+			return result(elem._type == type)
+		end, rest, nil
+	end
+end
+
+local function intersect_boxes(a_boxes, b_boxes)
+	local new_boxes = {}
+
+	for _, box_a in pairs(a_boxes) do
+		for _, box_b in pairs(b_boxes) do
+			-- Two boxes can only be merged if they're the same box or if one
+			-- or both selectors hasn't specified a box yet.
+			if box_a.name == nil or box_b.name == nil or box_a.name == box_b.name then
+				-- Create the new box by taking the specified box (if there is
+				-- one) and ORing the states together (making them more refer
+				-- to a more specific state).
+				local new_box = make_box(
+					box_a.name or box_b.name,
+					bit.bor(box_a.states, box_b.states)
+				)
+
+				-- Hash this box and add it into the table. This will be
+				-- effectively a no-op if there's already an identical box
+				-- hashed in the table.
+				new_boxes[hash_box(new_box)] = new_box
+			end
+		end
+	end
+
+	return new_boxes
+end
+
+function ui._intersect_sels(sels)
+	return function(elem)
+		-- We start with the default box, and intersect the box and states from
+		-- every selector with it.
+		local all_boxes = make_hashed()
+
+		-- Loop through all of the selectors. All of them need to match for the
+		-- intersected selector to match.
+		for _, sel in ipairs(sels) do
+			local matches, boxes = sel(elem)
+			if not matches then
+				-- This selector doesn't match, so fail immediately.
+				return false, nil
+			end
+
+			-- Since the selector matched, intersect the boxes and states with
+			-- those of the other selectors. If two selectors both match an
+			-- element but specify different boxes, then this selector will
+			-- return true, but the boxes will be cancelled out in the
+			-- intersection, leaving an empty list of boxes.
+			if boxes then
+				all_boxes = intersect_boxes(all_boxes, boxes)
+			end
+		end
+
+		return true, all_boxes
+	end
+end
+
+function ui._union_sels(sels)
+	return function(elem)
+		-- We initially have no boxes, and have to add them in as matching
+		-- selectors are unioned in.
+		local all_boxes = {}
+		local found_match = false
+
+		-- Loop through all of the selectors. If any of them match, this entire
+		-- unioned selector matches.
+		for _, sel in ipairs(sels) do
+			local matches, boxes = sel(elem)
+
+			if matches then
+				-- We found a match. However, we can't return true just yet
+				-- because we need to union the boxes and states from every
+				-- selector, not just this one.
+				found_match = true
+
+				if boxes then
+					-- Add the boxes from this selector into the table of all
+					-- the boxes. The hashing of boxes will automatically weed
+					-- out any duplicates.
+					for hash, box in pairs(boxes) do
+						all_boxes[hash] = box
+					end
+				end
+			end
+		end
+
+		if found_match then
+			return true, all_boxes
+		end
+		return false, nil
+	end
+end
+
+function ui._parse_sel(str, pred, partial)
+	str = str:trim()
+	assert(str ~= "", "Empty style selector")
+
+	local sub_sels = {}
+	local terms = {}
+	local done = false
+
+	-- Loop until we've read every term from the input string.
+	while not done do
+		-- Parse the next term from the input string.
+		local term, prefix
+		term, str, prefix = parse_term(str, pred)
+
+		-- If we read a term, insert this term into the list of terms for the
+		-- current sub-selector.
+		if term then
+			table.insert(terms, term)
+		end
+
+		-- Make sure that we have at least one selector term before each comma
+		-- or semicolon that we read.
+		if prefix then
+			assert(#terms > 0, "Expected selector term before '" .. prefix .. "'")
+		end
+
+		-- If we read a comma or semicolon or have run out of terms, we need to
+		-- commit the terms we've read so far.
+		if prefix or str == "" then
+			-- If there's only one term, commit it directly. Otherwise,
+			-- intersect all the terms together.
+			if #terms == 1 then
+				table.insert(sub_sels, terms[1])
+			else
+				table.insert(sub_sels, ui._intersect_sels(terms))
+			end
+
+			-- Clear out the list of terms for the next sub-selector.
+			terms = {}
+		end
+
+		-- If we read a semicolon or have run out of terms, we're done parsing.
+		-- We check for the semicolon case first since it is possible for the
+		-- string to be empty after reading the semicolon.
+		if prefix == ";" then
+			assert(partial, "Unexpected character ';' in selector")
+			done = true
+		elseif str == "" then
+			assert(prefix ~= ",", "Expected selector term after ','")
+			assert(not partial, "Expected ';' after end of selector")
+			done = true
+		end
+	end
+
+	-- Now that we've read all the sub-selectors between the commas, we need to
+	-- commit them. We only need to union the terms if there's more than one.
+	if #sub_sels == 1 then
+		return sub_sels[1], str:trim()
+	end
+	return ui._union_sels(sub_sels), str:trim()
+end
diff --git a/builtin/ui/static_elems.lua b/builtin/ui/static_elems.lua
new file mode 100644
index 000000000..35ffa79c4
--- /dev/null
+++ b/builtin/ui/static_elems.lua
@@ -0,0 +1,23 @@
+-- Luanti
+-- SPDX-License-Identifier: LGPL-2.1-or-later
+-- Copyright (C) 2024 v-rob, Vincent Robinson <robinsonvincent89@gmail.com>
+
+ui.Group = ui.derive_elem(ui.Elem, "group")
+ui.Label = ui.derive_elem(ui.Elem, "label")
+ui.Image = ui.derive_elem(ui.Elem, "image")
+
+ui.Root = ui._new_type(ui.Elem, "root", 0x01, false)
+
+function ui.Root:_init(props)
+	ui.Elem._init(self, props)
+
+	self._boxes.backdrop = true
+end
+
+function ui.Root:_encode_fields()
+	local fl = ui._make_flags()
+
+	self:_encode_box(fl, self._boxes.backdrop)
+
+	return ui._encode("SZ", ui.Elem._encode_fields(self), ui._encode_flags(fl))
+end
diff --git a/builtin/ui/style.lua b/builtin/ui/style.lua
new file mode 100644
index 000000000..767702964
--- /dev/null
+++ b/builtin/ui/style.lua
@@ -0,0 +1,281 @@
+-- Luanti
+-- SPDX-License-Identifier: LGPL-2.1-or-later
+-- Copyright (C) 2023 v-rob, Vincent Robinson <robinsonvincent89@gmail.com>
+
+ui.Style = core.class()
+
+function ui.Style:new(param)
+	local function make_style(props)
+		self:_init(ui._req(props, "table"))
+		return self
+	end
+
+	if type(param) == "string" then
+		self._sel = ui._parse_sel(param, false, false)
+		return make_style
+	end
+
+	self._sel = ui._universal_sel
+	return make_style(param)
+end
+
+function ui.Style:_init(props)
+	self._props = ui._cascade_props(props.props or props, {})
+	self._nested = table.merge(ui._opt_array(props.nested, ui.Style, props))
+	self._reset = ui._opt(props.reset, "boolean")
+
+	for _, item in ipairs(props) do
+		ui._req(item, ui.Style)
+	end
+end
+
+function ui.Style:_get_flat()
+	local flat_styles = {}
+	self:_get_flat_impl(flat_styles, ui._universal_sel)
+	return flat_styles
+end
+
+function ui.Style:_get_flat_impl(flat_styles, parent_sel)
+	-- Intersect our selector with our parent selector, resulting in a fully
+	-- qualified selector.
+	local full_sel = ui._intersect_sels({parent_sel, self._sel})
+
+	-- Copy this style's properties into a new style with the full selector.
+	local flat = ui.Style {
+		props = self._props,
+		reset = self._reset,
+	}
+	flat._sel = full_sel
+
+	table.insert(flat_styles, flat)
+
+	-- For each sub-style of this style, cascade it with our full selector and
+	-- add it to the list of flat styles.
+	for _, nested in ipairs(self._nested) do
+		nested:_get_flat_impl(flat_styles, full_sel)
+	end
+end
+
+local layout_type_map = {
+	place = 0,
+}
+
+local dir_flags_map = {
+	none = 0,
+	x = 1,
+	y = 2,
+	both = 3,
+}
+
+local display_mode_map = {
+	visible = 0,
+	overflow = 1,
+	hidden = 2,
+	clipped = 3,
+}
+
+local icon_place_map = {
+	center = 0,
+	left = 1,
+	top = 2,
+	right = 3,
+	bottom = 4,
+}
+
+local function opt_color(val, def)
+	assert(val == nil or core.colorspec_to_int(val))
+	return val or def
+end
+
+local function opt_vec2d(val, def)
+	ui._opt_array(val, "number")
+	assert(val == nil or #val == 1 or #val == 2)
+	return val or def
+end
+
+local function opt_rect(val, def)
+	ui._opt_array(val, "number")
+	assert(val == nil or #val == 1 or #val == 2 or #val == 4)
+	return val or def
+end
+
+local function cascade_layout(new, add, props)
+	new.layout = ui._opt_enum(add.layout, layout_type_map, props.layout)
+	new.clip = ui._opt_enum(add.clip, dir_flags_map, props.clip)
+
+	new.scale = ui._opt(add.scale, "number", props.scale)
+end
+
+local function cascade_sizing(new, add, props)
+	new.size = opt_vec2d(add.size, props.size)
+	new.span = opt_vec2d(add.span, props.span)
+
+	new.pos = opt_vec2d(add.pos, props.pos)
+	new.anchor = opt_vec2d(add.anchor, props.anchor)
+
+	new.margin = opt_rect(add.margin, props.margin)
+	new.padding = opt_rect(add.padding, props.padding)
+end
+
+local function cascade_layer(new, add, props, p)
+	new[p.."_image"] = ui._opt(add[p.."_image"], "string", props[p.."_image"])
+	new[p.."_fill"] = opt_color(add[p.."_fill"], props[p.."_fill"])
+	new[p.."_tint"] = opt_color(add[p.."_tint"], props[p.."_tint"])
+
+	new[p.."_scale"] = ui._opt(add[p.."_scale"], "number", props[p.."_scale"])
+	new[p.."_source"] = opt_rect(add[p.."_source"], props[p.."_source"])
+
+	new[p.."_frames"] = ui._opt(add[p.."_frames"], "number", props[p.."_frames"])
+	new[p.."_frame_time"] =
+			ui._opt(add[p.."_frame_time"], "number", props[p.."_frame_time"])
+end
+
+function ui._cascade_props(add, props)
+	local new = {}
+
+	cascade_layout(new, add, props)
+	cascade_sizing(new, add, props)
+
+	new.display = ui._opt_enum(add.display, display_mode_map, props.display)
+
+	cascade_layer(new, add, props, "box")
+	cascade_layer(new, add, props, "icon")
+
+	new.box_middle = opt_rect(add.box_middle, props.box_middle)
+	new.box_tile = ui._opt_enum(add.box_tile, dir_flags_map, props.box_tile)
+
+	new.icon_place = ui._opt_enum(add.icon_place, icon_place_map, props.icon_place)
+	new.icon_gutter = ui._opt(add.icon_gutter, "number", props.icon_gutter)
+	new.icon_overlap = ui._opt(add.icon_overlap, "boolean", props.icon_overlap)
+
+	return new
+end
+
+local function unpack_vec2d(vec)
+	if #vec == 2 then
+		return vec[1], vec[2]
+	elseif #vec == 1 then
+		return vec[1], vec[1]
+	end
+end
+
+local function unpack_rect(rect)
+	if #rect == 4 then
+		return rect[1], rect[2], rect[3], rect[4]
+	elseif #rect == 2 then
+		return rect[1], rect[2], rect[1], rect[2]
+	elseif #rect == 1 then
+		return rect[1], rect[1], rect[1], rect[1]
+	end
+end
+
+local function encode_layout(props)
+	local fl = ui._make_flags()
+
+	if ui._shift_flag(fl, props.layout) then
+		ui._encode_flag(fl, "B", layout_type_map[props.layout])
+	end
+	if ui._shift_flag(fl, props.clip) then
+		ui._encode_flag(fl, "B", dir_flags_map[props.clip])
+	end
+
+	if ui._shift_flag(fl, props.scale) then
+		ui._encode_flag(fl, "f", props.scale)
+	end
+
+	return fl
+end
+
+local function encode_sizing(props)
+	local fl = ui._make_flags()
+
+	if ui._shift_flag(fl, props.size) then
+		ui._encode_flag(fl, "ff", unpack_vec2d(props.size))
+	end
+	if ui._shift_flag(fl, props.span) then
+		ui._encode_flag(fl, "ff", unpack_vec2d(props.span))
+	end
+
+	if ui._shift_flag(fl, props.pos) then
+		ui._encode_flag(fl, "ff", unpack_vec2d(props.pos))
+	end
+	if ui._shift_flag(fl, props.anchor) then
+		ui._encode_flag(fl, "ff", unpack_vec2d(props.anchor))
+	end
+
+	if ui._shift_flag(fl, props.margin) then
+		ui._encode_flag(fl, "ffff", unpack_rect(props.margin))
+	end
+	if ui._shift_flag(fl, props.padding) then
+		ui._encode_flag(fl, "ffff", unpack_rect(props.padding))
+	end
+
+	return fl
+end
+
+local function encode_layer(props, p)
+	local fl = ui._make_flags()
+
+	if ui._shift_flag(fl, props[p.."_image"]) then
+		ui._encode_flag(fl, "z", props[p.."_image"])
+	end
+	if ui._shift_flag(fl, props[p.."_fill"]) then
+		ui._encode_flag(fl, "I", core.colorspec_to_int(props[p.."_fill"]))
+	end
+	if ui._shift_flag(fl, props[p.."_tint"]) then
+		ui._encode_flag(fl, "I", core.colorspec_to_int(props[p.."_tint"]))
+	end
+
+	if ui._shift_flag(fl, props[p.."_scale"]) then
+		ui._encode_flag(fl, "f", props[p.."_scale"])
+	end
+	if ui._shift_flag(fl, props[p.."_source"]) then
+		ui._encode_flag(fl, "ffff", unpack_rect(props[p.."_source"]))
+	end
+
+	if ui._shift_flag(fl, props[p.."_frames"]) then
+		ui._encode_flag(fl, "I", props[p.."_frames"])
+	end
+	if ui._shift_flag(fl, props[p.."_frame_time"]) then
+		ui._encode_flag(fl, "I", props[p.."_frame_time"])
+	end
+
+	return fl
+end
+
+local function encode_subflags(fl, sub_fl)
+	if ui._shift_flag(fl, sub_fl.flags ~= 0) then
+		ui._encode_flag(fl, "s", ui._encode_flags(sub_fl))
+	end
+end
+
+function ui._encode_props(props)
+	local fl = ui._make_flags()
+
+	encode_subflags(fl, encode_layout(props))
+	encode_subflags(fl, encode_sizing(props))
+
+	if ui._shift_flag(fl, props.display) then
+		ui._encode_flag(fl, "B", display_mode_map[props.display])
+	end
+
+	encode_subflags(fl, encode_layer(props, "box"))
+	encode_subflags(fl, encode_layer(props, "icon"))
+
+	if ui._shift_flag(fl, props.box_middle) then
+		ui._encode_flag(fl, "ffff", unpack_rect(props.box_middle))
+	end
+	if ui._shift_flag(fl, props.box_tile) then
+		ui._encode_flag(fl, "B", dir_flags_map[props.box_tile])
+	end
+
+	if ui._shift_flag(fl, props.icon_place) then
+		ui._encode_flag(fl, "B", icon_place_map[props.icon_place])
+	end
+	if ui._shift_flag(fl, props.icon_gutter) then
+		ui._encode_flag(fl, "f", props.icon_gutter)
+	end
+	ui._shift_flag_bool(fl, props.icon_overlap)
+
+	return ui._encode("s", ui._encode_flags(fl))
+end
diff --git a/builtin/ui/theme.lua b/builtin/ui/theme.lua
new file mode 100644
index 000000000..9aa761335
--- /dev/null
+++ b/builtin/ui/theme.lua
@@ -0,0 +1,36 @@
+-- Luanti
+-- SPDX-License-Identifier: LGPL-2.1-or-later
+-- Copyright (C) 2023 v-rob, Vincent Robinson <robinsonvincent89@gmail.com>
+
+local prelude_theme = ui.Style {
+	ui.Style "root" {
+		pos = {1/2},
+		anchor = {1/2},
+		span = {0},
+
+		ui.Style "@backdrop" {
+			display = "hidden",
+			clip = "both",
+		},
+		ui.Style "@backdrop$focused" {
+			display = "visible",
+		},
+	},
+	ui.Style "image" {
+		icon_scale = 0,
+	},
+}
+
+function ui.get_prelude_theme()
+	return prelude_theme
+end
+
+local default_theme = prelude_theme
+
+function ui.get_default_theme()
+	return default_theme
+end
+
+function ui.set_default_theme(theme)
+	default_theme = ui._req(theme, ui.Style)
+end
diff --git a/builtin/ui/util.lua b/builtin/ui/util.lua
new file mode 100644
index 000000000..25b8a922e
--- /dev/null
+++ b/builtin/ui/util.lua
@@ -0,0 +1,120 @@
+-- Luanti
+-- SPDX-License-Identifier: LGPL-2.1-or-later
+-- Copyright (C) 2023 v-rob, Vincent Robinson <robinsonvincent89@gmail.com>
+
+local next_id = 0
+
+function ui.new_id()
+	-- Just increment a monotonic counter and return it as hex. Even at
+	-- unreasonably fast ID generation rates, it would take years for this
+	-- counter to hit the 2^53 limit and start generating duplicates.
+	next_id = next_id + 1
+	return string.format("_%X", next_id)
+end
+
+ui._ID_CHARS = "a-zA-Z0-9_%-%:"
+
+function ui.is_id(str)
+	return type(str) == "string" and str == str:match("^[" .. ui._ID_CHARS .. "]+$")
+end
+
+function ui._is_reserved_id(str)
+	return ui.is_id(str) and str:match("^[_-]")
+end
+
+-- This coordinate size calculation copies the one for fixed-size formspec
+-- coordinates in guiFormSpecMenu.cpp.
+function ui.get_coord_size()
+	return math.floor(0.5555 * 96)
+end
+
+function ui._req(val, typ)
+	assert(type(val) == typ or
+			(typ == "id" and ui.is_id(val)) or core.is_instance(val, typ))
+	return val
+end
+
+function ui._opt(val, typ, def)
+	if val == nil then
+		return def
+	end
+	return ui._req(val, typ)
+end
+
+function ui._req_array(arr, typ)
+	for _, val in ipairs(ui._req(arr, "table")) do
+		ui._req(val, typ)
+	end
+	return arr
+end
+
+function ui._opt_array(arr, typ, def)
+	for _, val in ipairs(ui._opt(arr, "table", {})) do
+		ui._req(val, typ)
+	end
+	return arr or def
+end
+
+function ui._req_enum(val, enum)
+	assert(type(val) == "string" and enum[val])
+	return val
+end
+
+function ui._opt_enum(val, enum, def)
+	if val == nil then
+		return def
+	end
+	return ui._req_enum(val, enum)
+end
+
+ui._encode = core.encode_network
+ui._decode = core.decode_network
+
+function ui._encode_array(format, arr)
+	local formatted = {}
+	for _, val in ipairs(arr) do
+		table.insert(formatted, ui._encode(format, val))
+	end
+
+	return ui._encode("IZ", #formatted, table.concat(formatted))
+end
+
+function ui._pack_flags(...)
+	local flags = 0
+	for _, flag in ipairs({...}) do
+		flags = bit.bor(bit.lshift(flags, 1), flag and 1 or 0)
+	end
+	return flags
+end
+
+function ui._make_flags()
+	return {flags = 0, num_flags = 0, data = {}}
+end
+
+function ui._shift_flag(fl, flag)
+	-- OR the LSB with the condition, and then right rotate it to the MSB.
+	fl.flags = bit.ror(bit.bor(fl.flags, flag and 1 or 0), 1)
+	fl.num_flags = fl.num_flags + 1
+
+	return flag
+end
+
+function ui._shift_flag_bool(fl, flag)
+	if ui._shift_flag(fl, flag ~= nil) then
+		ui._shift_flag(fl, flag)
+	else
+		ui._shift_flag(fl, false)
+	end
+end
+
+function ui._encode_flag(fl, ...)
+	table.insert(fl.data, ui._encode(...))
+end
+
+function ui._encode_flags(fl)
+	-- We've been shifting into the right the entire time, so flags are in the
+	-- upper bits; however, the protocol expects them to be in the lower bits.
+	-- So, shift them the appropriate amount into the lower bits.
+	local adjusted = bit.rshift(fl.flags, 32 - fl.num_flags)
+	return ui._encode("I", adjusted) .. table.concat(fl.data)
+end
diff --git a/builtin/ui/window.lua b/builtin/ui/window.lua
new file mode 100644
index 000000000..8da479f64
--- /dev/null
+++ b/builtin/ui/window.lua
@@ -0,0 +1,190 @@
+-- Luanti
+-- SPDX-License-Identifier: LGPL-2.1-or-later
+-- Copyright (C) 2023 v-rob, Vincent Robinson <robinsonvincent89@gmail.com>
+
+ui.Window = core.class()
+
+ui._window_types = {
+	filter = 0,
+	mask = 1,
+	hud = 2,
+	chat = 3,
+	gui = 4,
+}
+
+function ui.Window:new(param)
+	local function make_window(props)
+		self:_init(ui._req(props, "table"))
+		return self
+	end
+
+	self._type = ui._req_enum(param, ui._window_types)
+	return make_window
+end
+
+function ui.Window:_init(props)
+	self._theme = ui._opt(props.theme, ui.Style, ui.get_default_theme())
+	self._styles = ui._opt_array(props.styles, ui.Style, props)
+
+	self._root = ui._req(props.root, ui.Root)
+
+	self._context = nil -- Set by ui.Context
+
+	self._elems = self._root:_get_flat()
+	self._elems_by_id = {}
+
+	for _, elem in ipairs(self._elems) do
+		local id = elem._id
+
+		assert(not self._elems_by_id[id], "Element has duplicate ID: '" .. id .. "'")
+		self._elems_by_id[id] = elem
+
+		assert(elem._window == nil, "Element '" .. elem._id .. "' already has a window")
+		elem._window = self
+	end
+
+	for _, item in ipairs(props) do
+		ui._req(item, ui.Style)
+	end
+end
+
+function ui.Window:_encode(player, opening)
+	local enc_styles = self:_encode_styles()
+	local enc_elems = self:_encode_elems()
+
+	local data = ui._encode("ZzZ", enc_elems, self._root._id, enc_styles)
+	if opening then
+		data = ui._encode("ZB", data, ui._window_types[self._type])
+	end
+
+	return data
+end
+
+function ui.Window:_encode_styles()
+	-- Clear out all the boxes in every element.
+	for _, elem in ipairs(self._elems) do
+		for box in pairs(elem._boxes) do
+			elem._boxes[box] = {n = 0}
+		end
+	end
+
+	-- Get a cascaded and flattened list of all the styles for this window.
+	local styles = self:_get_full_style():_get_flat()
+
+	-- Take each style and apply its properties to every box and state matched
+	-- by its selector.
+	self:_apply_styles(styles)
+
+	-- Take the styled boxes and encode their styles into a single table,
+	-- replacing the boxes' style property tables with indices into this table.
+	local enc_styles = self:_index_styles()
+
+	return ui._encode_array("Z", enc_styles)
+end
+
+function ui.Window:_get_full_style()
+	-- The full style contains the theme, global styles, and local element
+	-- styles as sub-styles, in that order, to ensure the correct precedence.
+	local styles = table.merge({self._theme}, self._styles)
+
+	for _, elem in ipairs(self._elems) do
+		-- Cascade the inline style with the element's ID, ensuring that the
+		-- inline style globally refers to this element only.
+		local local_style = ui.Style("#" .. elem._id) {
+			props = elem._props,
+			nested = elem._styles,
+		}
+		table.insert(styles, local_style)
+	end
+
+	-- Return all these styles wrapped up into a single style.
+	return ui.Style {nested = styles}
+end
+
+local function apply_style(elem, boxes, style)
+	-- Loop through each box, applying the styles accordingly. The table of
+	-- boxes may be empty, in which case nothing happens.
+	for _, box in pairs(boxes) do
+		local name = box.name or "main"
+
+		-- If this style resets all properties, find all states that are a
+		-- subset of the state being styled and clear their property tables.
+		if style._reset then
+			for i = ui._STATE_NONE, ui._NUM_STATES - 1 do
+				if bit.band(box.states, i) == box.states then
+					elem._boxes[name][i] = nil
+				end
+			end
+		end
+
+		-- Get the existing style property table for this box if it exists.
+		local props = elem._boxes[name][box.states] or {}
+
+		-- Cascade the properties from this style onto the box.
+		elem._boxes[name][box.states] = ui._cascade_props(style._props, props)
+	end
+end
+
+function ui.Window:_apply_styles(styles)
+	-- Loop through each style and element and see if the style properties can
+	-- be applied to any boxes.
+	for _, style in ipairs(styles) do
+		for _, elem in ipairs(self._elems) do
+			-- Check if the selector for this style. If it matches, apply the
+			-- style to each of the applicable boxes.
+			local matches, boxes = style._sel(elem)
+			if matches then
+				apply_style(elem, boxes, style)
+			end
+		end
+	end
+end
+
+local function index_style(box, i, style_indices, enc_styles)
+	-- If we have a style for this state, serialize it to a string. Identical
+	-- styles have identical strings, so we use this to our advantage.
+	local enc = ui._encode_props(box[i])
+
+	-- If we haven't serialized a style identical to this one before, store
+	-- this as the latest index in the list of style strings.
+	if not style_indices[enc] then
+		style_indices[enc] = #enc_styles
+		table.insert(enc_styles, enc)
+	end
+
+	-- Set the index of our state to the index of its style string, and keep
+	-- count of how many states with valid indices we have for this box so far.
+	box[i] = style_indices[enc]
+	box.n = box.n + 1
+end
+
+function ui.Window:_index_styles()
+	local style_indices = {}
+	local enc_styles = {}
+
+	for _, elem in ipairs(self._elems) do
+		for _, box in pairs(elem._boxes) do
+			for i = ui._STATE_NONE, ui._NUM_STATES - 1 do
+				if box[i] then
+					-- If this box has a style, encode and index it.
+					index_style(box, i, style_indices, enc_styles)
+				else
+					-- Otherwise, this state has no style, so set it as such.
+					box[i] = ui._NO_STYLE
+				end
+			end
+		end
+	end
+
+	return enc_styles
+end
+
+function ui.Window:_encode_elems()
+	local enc_elems = {}
+
+	for _, elem in ipairs(self._elems) do
+		table.insert(enc_elems, elem:_encode())
+	end
+
+	return ui._encode_array("Z", enc_elems)
+end
diff --git a/doc/lua_api.md b/doc/lua_api.md
index 100a75f0c..81a9ee495 100644
--- a/doc/lua_api.md
+++ b/doc/lua_api.md
@@ -4237,6 +4237,8 @@ Helper functions
     * since 5.12
     * `table` can also be non-table value, which will be returned as-is
     * preserves metatables as they are
+    * Returns a deep copy of `table`, i.e. a copy of the table and all its
+      nested tables.
 * `table.indexof(list, val)`: returns the smallest numerical index containing
       the value `val` in the table `list`. Non-numerical indices are ignored.
       If `val` could not be found, `-1` is returned. `list` must not have
@@ -4248,6 +4250,9 @@ Helper functions
 * `table.insert_all(table, other_table)`:
     * Appends all values in `other_table` to `table` - uses `#table + 1` to
       find new indices.
+* `table.merge(...)`:
+    * Merges multiple tables together into a new single table using
+      `table.insert_all()`.
 * `table.key_value_swap(t)`: returns a table with keys and values swapped
     * If multiple keys in `t` map to the same value, it is unspecified which
       value maps to that key.
@@ -5993,6 +5998,63 @@ Utilities
 * `core.urlencode(str)`: Encodes reserved URI characters by a
   percent sign followed by two hex digits. See
   [RFC 3986, section 2.3](https://datatracker.ietf.org/doc/html/rfc3986#section-2.3).
+* `core.class([super])`: Creates a new metatable-based class.
+    * `super` (optional): The superclass of the newly created class, or nil if
+      the class should not have a superclass.
+    * The class table is given a metatable with an `__index` field that points
+      to `super` and a `__call` metamethod that constructs a new object.
+    * By default, the only field in the class table is an `__index` field that
+      points to itself. When an instance of the class is created, the class
+      table is set as the metatable for the object.
+    * When a new object is constructed via `__call`, the `new()` method will be
+      called if it exists. If `new()` returns a nonzero number of values, then
+      those values will be returned from `__call`. Otherwise, if `new()`
+      returned no values, the object iself will be returned.
+    * Extra Lua metamethods like `__add` may be added to the class table, but
+      note that these fields will not be inherited by subclasses since Lua
+      doesn't consult `__index` when searching for metamethods.
+    * Example: The following code, demonstrating a simple example of classes
+      and inheritance, will print `Rectangle[area=6, filled=true]`:
+      ```lua
+      local Shape = core.class()
+      Shape.name = "Shape"
+
+      function Shape:new(filled)
+          self.filled = filled
+      end
+
+      function Shape:describe()
+          return string.format("%s[area=%d, filled=%s]",
+                  self.name, self:get_area(), self.filled)
+      end
+
+      local Rectangle = core.class(Shape)
+      Rectangle.name = "Rectangle"
+
+      function Rectangle:new(filled, width, height)
+          Shape.new(self, filled)
+
+          self.width = width
+          self.height = height
+      end
+
+      function Rectangle:get_area()
+          return self.width * self.height
+      end
+
+      local shape = Rectangle(true, 2, 3)
+      print(shape:describe())
+
+      assert(core.is_instance(shape, Shape))
+      assert(core.is_subclass(Rectangle, Shape))
+      assert(core.super(Rectangle) == Shape)
+      ```
+* `core.super(class)`: Returns the superclass of `class`, or nil if the table
+  is not a class or has no superclass.
+* `core.is_subclass(class, super)`: Returns true if `class` is a subclass of
+  `super`.
+* `core.is_instance(obj, class)`: Returns true if `obj` is an instance of
+  `class` or any of its subclasses.
 
 Logging
 -------