rust_browser/docs/css_support_matrix.md

CSS Support Matrix

This document tracks implemented CSS features and gaps, with a focus on completing CSS 2.1 support. CSS is modular (there is no single “CSS4”), so this is organized by selectors, properties, values, and at-rules as implemented in this codebase.

Scope And Prioritization

Prioritization is based on visual impact and popularity in typical pages.

Implemented Today (High-Level)

Selectors:

• Type, class, ID, universal selectors
• Attribute selectors: [attr], [attr=value], ^=, $=, *=, ~=, |=
• Combinators: descendant, child (>), adjacent sibling (+), general sibling (~)
• Pseudo-classes: :root, :link, :checked, :disabled, :enabled, :required, :optional, :read-only, :read-write, :defined, :dir(), :lang(), :not(), :is(), :where(), :has(), :first-child, :last-child, :only-child, :nth-child(), :nth-last-child(), :first-of-type, :last-of-type, :only-of-type, :nth-of-type(), :nth-last-of-type(), :empty

Properties (parsed + applied):

• display (block, inline, inline-block, flex, none, table variants)
• Box model: width, height, margin-*, padding-*, border-*-width, border-*-color, border-*-style
• Shorthands: margin, padding, border, border-*, border-width, border-color, border-style, background (color only)
• background-color, color, font-size
• Positioning: position, top, right, bottom, left, z-index
• Overflow: overflow, overflow-x, overflow-y (visible, hidden)
• Flexbox: flex-direction, justify-content, align-items, gap, flex-grow, flex-shrink, flex-basis
• Text: white-space, line-height, text-align
• Tables: border-spacing, vertical-align
• Constraints: max-width, min-width, max-height, min-height
• Floats: float, clear
• Custom properties (CSS variables), including var() in key shorthands

Values:

• Length units: px, em, rem, %
• Colors: named colors, hex (#rgb, #rgba, #rrggbb, #rrggbbaa), rgb(), rgba()

At-rules:

• None (all @ rules are skipped)

Known Missing Or Incomplete Behavior

Selectors and pseudo-classes:

• Pseudo-elements never match (e.g., ::before, ::after, ::first-line, ::first-letter, ::placeholder).
• Many pseudo-classes are parsed but always return false: :hover, :focus, :focus-visible, :focus-within, :active, :visited, :target, :placeholder-shown, :default, :valid, :invalid, :in-range, :out-of-range, :indeterminate.
• Attribute selector case-sensitivity flags (i/s) are parsed but not honored.

At-rules and media:

• @media is supported (viewport width/height, prefers-color-scheme; nesting supported).
• @font-face is supported (font-family, src with url()/format(), font-weight, font-style; TTF/OTF only, WOFF2 not yet supported).
• @import, @supports, @keyframes, @layer are not supported.

Properties and values:

• Many CSS 2.1 properties are missing entirely (see Prioritized Backlog below).
• background shorthand extracts color and linear-gradient() (no repeat, position, size).
• Border styles are limited to none, solid, dashed, dotted.
• Units missing: pt, pc, in, cm, mm, ex, ch.
• Color functions missing: hsl(), hsla().

Prioritized Backlog For CSS 2.1 Completion

Tier 1: High visual impact and common usage

• font-weight (bold text for <b>, <strong>, <th>)
• font-style (italics for <em>, <i>, <cite>, <var>)
• font-family (monospace for <code>, <pre>, <kbd>, <samp>)
• text-decoration (underlines for links)
• list-style-type, list-style-position (bulleted and numbered lists)
• background-image with url() and linear-gradient() (including -webkit- and -o- prefixes), background-repeat, background-position (basic background rendering)
• Pseudo-elements ::before and ::after (even limited content support for simple strings)
• :hover and :focus (core interactive styling)

Tier 2: Layout correctness and common rendering expectations

• border-collapse (tables)
• caption-side (tables)
• vertical-align for inline/inline-block elements in layout (not just computed value)
• line-height inheritance nuances and normal default per font metrics (currently fixed 1.2)
• % resolution for height, top, bottom with containing block rules
• overflow: scroll and overflow: auto
• visibility (hidden while taking space)
• white-space layout behavior for pre-wrap and pre-line

Tier 3: CSS 2.1 coverage and polish

• text-transform
• letter-spacing, word-spacing
• text-indent
• direction, unicode-bidi
• quotes
• content (full support for pseudo-elements)
• outline properties
• cursor
• Full unit support: pt, pc, in, cm, mm, ex, ch
• Full color support: hsl(), hsla()

Notes

• This list reflects observed gaps in crates/css, crates/style, and crates/selectors as of the current tree.
• If you add a property, update the parser (PropertyId and parsing), computed style (ComputedStyles), and layout/paint as needed.