Status: Partial — core properties (font-family, font-size, font-weight, font-style, font-stretch, font-variant small-caps) and matching are implemented; font shorthand, font-feature-settings, variable fonts, and unicode-range remain unimplemented. Standard: CSS Fonts Level 4 Related: text/0052-overview.md

Summary

CSS Fonts Level 4 defines font selection, loading, and rendering properties. SVG uses these as presentation attributes on text elements. This document tracks our support level and implementation plan for CSS font properties within the donner SVG renderer.

Current Support

Implemented Properties

Property	Status	Notes
font-family	Implemented	Parsed as comma-separated list. Resolved via FontManager::findFont() against registered @font-face rules. Falls back to embedded Public Sans.
font-size	Implemented	Supports px, em, %, and other CSS length units, absolute-size keywords (xx-small–xx-large), and larger/smaller relative keywords. Inherited.
font-weight	Implemented	Supports normal (400), bold (700), numeric 100-900, and bolder/lighter relative keywords. Per-span resolution selects weight-matched font from registered @font-face rules.
font-style	Implemented	normal, italic, oblique parsed (PropertyRegistry) and used in FontManager::findFont(family, weight, style, stretch) matching.
font-stretch	Implemented	Keyword and percentage values parsed; used in font matching.
font-variant	Partial	normal/small-caps parsed; small-caps synthesized in the base tier, native smcp OpenType feature used in the full tier. Other CSS Fonts 4 sub-values (all-small-caps, petite-caps, etc.) not supported.

Registered but Not Parsed (stored as unparsed presentation attributes)

Property	CSS Fonts 4 Section	Priority	Notes
font-size-adjust	3.5	Low	Adjusts x-height across font fallback. Complex metric computation.

Not Registered

Property	CSS Fonts 4 Section	Priority	Notes
font	3.7	Medium	Shorthand (font: bold 16px/1.2 "Noto Sans"). Common in CSS stylesheets.
font-synthesis	3.8	Low	Controls synthetic bold/italic. Not commonly used in SVG.
font-feature-settings	6.12	Medium	Direct OpenType feature control ("liga" 0, "smcp" 1). Passes to HarfBuzz hb_feature_t.
font-variation-settings	5.2	Low	Variable font axis values. Requires variable font support.
font-optical-sizing	5.3	Low	Automatic optical sizing for variable fonts.
font-palette	7.1	Low	Color font palette selection.
line-height	Related (CSS Inline 3)	Medium	Not in CSS Fonts 4 but essential for multi-line text.

@font-face Support

Current State

@font-face rules are supported with the following descriptors:

Descriptor	Status	Notes
font-family	Implemented	Family name for matching.
src	Partial	Data (inline bytes) supported. url() and local() not implemented.
font-weight	Implemented	Stored in FontFace::fontWeight. Weight matching in findFont(family, weight).
font-style	Implemented	Stored in FontFace::fontStyle. Style matching in findFont(family, weight, style, stretch).
font-stretch	Implemented	Stored on FontFace. Stretch matching in findFont(family, weight, style, stretch).
font-display	Not applicable	Only relevant for web loading behavior.
unicode-range	Not implemented	Would enable per-codepoint font fallback.

Font Matching Algorithm

CSS Fonts 4 Section 4.7 defines a multi-step matching algorithm:

Family — match by font-family name
Style — prefer matching font-style (italic, oblique, normal)
Weight — closest numeric weight match
Stretch — closest width match

Our current implementation:

Step 1: ✅ Exact family name match
Step 2: ✅ Style matching (findFont(family, weight, style, stretch))
Step 3: ✅ Basic closest-weight match via abs(face.fontWeight - weight)
Step 4: ✅ Stretch matching

Implementation Plan

Phase 1 (font-style) is complete: FontStyle is a PropertyRegistry property and a FontFace descriptor, and FontManager::findFont(family, weight, style, stretch) matches on it.

Phase 2: font shorthand (Medium Priority)

Parse the font shorthand property which combines: font: [font-style] [font-variant] [font-weight] font-size[/line-height] font-family

Example: font: bold 16px "Noto Sans" → weight=700, size=16px, family="Noto Sans".

Phase 3: font-feature-settings (Medium Priority)

Pass OpenType feature settings to HarfBuzz via hb_feature_t. Parse format: font-feature-settings: "liga" 0, "smcp" 1

TextBackendFull changes: Store parsed features in TextLayoutParams and pass to hb_shape():

hb_shape(hbFont, chunkBuf, features.data(), features.size());

Phase 4: Variable fonts (Low Priority)

Support font-variation-settings for variable font axes (wght, wdth, ital, etc.). Requires FreeType's FT_Set_Var_Design_Coordinates API.

Font Fallback

Current Implementation

Per-script font fallback: when the primary font lacks glyphs for a script (checked via FT_Get_Char_Index), iterates all registered @font-face rules to find one with coverage. This handles cases like "Noto Sans" requested for Arabic text falling back to Amiri.

Planned Improvements

unicode-range matching — CSS Fonts 4 @font-face unicode-range descriptor enables per-codepoint font selection without probing each font's cmap.
Per-glyph fallback — Currently fallback is per-span (entire span uses the fallback font). Per-glyph fallback would allow mixed-script text within a single span, splitting into font runs.
System font access — Currently only @font-face registered fonts are available. System font enumeration would enable local() font sources.

Resvg Test Coverage

Font-related resvg status in the full-text tier (--config=text-full). Living reference: regenerate with bazel test //donner/svg/renderer/tests:resvg_test_suite_max and read bazel-testlogs/.../resvg_test_suite_max/test.xml (enabled = status="run", disabled = status="notrun"). The suite is green, so every enabled test passes.

Test Group	Enabled passing	Disabled	Notes
a-font-weight-*	12/12	0	Now passing at default params (the old 18K-threshold override is gone)
a-font-size-*	20/20	0	Keyword + relative-unit support landed; named-value{,-without-a-parent} pass via blessed goldens (CSS Fonts L4 differs from resvg — see 0009)
e-tspan-*	24/24	7	e-tspan-006 (bold) and e-tspan-024 (bold + text-anchor) now pass; e-tspan-028 (mixed font-size) is among the 7 disabled