docs(htmlcss): expand CSS property reference and overhaul L0 fixtures

.agents/skills/fixtures/SKILL.md

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+---
+description: Guides authoring, organizing, and referencing test fixtures across the project. Use when creating new fixtures, writing tests that depend on fixtures, or deciding what should be checked into git.
+---
+
+# Fixtures
+
+## What fixtures are
+
+Fixtures are static input files — HTML, SVG, CSS, JSON, `.grida`, `.fig`,
+font binaries, images, text samples — used as **deterministic inputs** to
+rendering, parsing, and I/O tests. They exist so that tests are reproducible,
+self-contained, and don't depend on external services or generated data.
+
+## Why we keep them
+
+- **Regression detection** — render the same input, compare the output.
+- **Spec coverage** — each fixture maps to a specific feature or property
+  being tested (one concept per file).
+- **Onboarding** — new contributors can see exactly what the renderer handles
+  by browsing fixtures.
+- **Cross-pipeline validation** — the same fixture can be consumed by unit
+  tests, golden tests, reftests, probe tests, and visual inspection.
+
+## What should be covered
+
+A fixture should exist for every **rendering behavior, format variant, or
+edge case** that the codebase supports or intends to support. This includes:
+
+- Each CSS property / SVG element the htmlcss or SVG renderer handles
+- Format I/O round-trips (Figma → Grida, SVG → Grida, clipboard paste)
+- Edge cases: zero-size, empty content, deeply nested, degenerate inputs
+- Unsupported-but-tracked features (the fixture documents the gap)
+
+## Best practices
+
+- **One concept per file.** Don't combine unrelated properties.
+- **Self-contained.** No external resources, network fetches, or scripts.
+- **Minimal.** Only enough structure to isolate the behavior under test.
+- **Probe-friendly.** High-contrast palette (prefer B/W), round pixel values,
+  ≤ 3 colors. Designed for headless pixel probing, not human aesthetics.
+- **Descriptive naming.** `<domain>-<property>[-<descriptor>].<ext>` — the
+  filename alone should tell you what's being tested.
+- **Labeled specimens.** Within a fixture, label each test case with the
+  value being exercised so both humans and heuristics can identify regions.
+- **Don't duplicate.** Before adding a fixture, check if an existing one
+  already covers the behavior. Extend or split rather than duplicate.
+
+## Git inclusion policy
+
+### Checked in (`fixtures/`)
+
+All directories under `fixtures/` **except `fixtures/local/`** are committed
+to the repository. These are small, purpose-built files that are part of the
+test suite.
+
+```
+fixtures/
+├── css/              # CSS stylesheets
+├── fonts/            # Bundled font binaries (deterministic text tests)
+├── images/           # Test images
+├── test-fig/         # Figma clipboard / REST fixtures
+├── test-figma/       # Figma archive fixtures
+├── test-grida/       # .grida format fixtures
+├── test-html/        # HTML+CSS renderer fixtures (L0, etc.)
+├── test-markdown/    # Markdown fixtures
+├── test-svg/         # SVG fixtures
+├── text/             # Plain text samples
+└── local/            # ← gitignored, see below
+```
+
+### Not checked in (`fixtures/local/`)
+
+`fixtures/local/` is **gitignored**. It holds large, third-party, or
+benchmark-only datasets that are meaningful for local development but too
+large or license-restricted for the repository:
+
+- `W3C_SVG_11_TestSuite` — W3C SVG 1.1 conformance suite (~50 MB)
+- `resvg-test-suite` — resvg's feature-focused SVG tests
+- `oxygen-icons-5.116.0` — icon set for stress testing
+- `perf` — large scenes for benchmarking
+
+These must be downloaded separately by developers who need them.
+
+## Referencing local-only fixtures
+
+**Never reference `fixtures/local/` paths in committed code, tests, or
+documentation.** Local fixtures do not exist in CI or on other developers'
+machines. Specifically:
+
+- Do not `include!()`, `read_to_string()`, or `fs::read()` a `local/` path
+  in any Rust test or example that runs in CI.
+- Do not hardcode `fixtures/local/` paths in docs, READMEs, or AGENTS files
+  as if they are always available.
+- If a doc needs to mention a local suite (e.g. for reftest instructions),
+  clearly mark it as **local-only** and note that the developer must download
+  it first.
+- Tests that depend on local fixtures must be gated (e.g. `#[ignore]` with a
+  comment, or behind a feature flag) so they don't fail in CI.