feat(css): add schema validation and supplementary documentation for custom properties

[jonathandewitt-dev]

Description

Adds patternProperties validation to the CSS properties schema to explicitly validate custom property names (CSS variables). Custom properties matching the pattern ^-- are now recognized as valid property keys in the schema.

Motivation

The schema currently validates properties through additionalProperties, which treats every key individually without recognizing patterns. While properties.json contains a "--*" entry to document custom properties, the schema doesn't express that any key starting with -- is valid (e.g., --brand-color, --spacing-md).

This creates several problems:

1. Semantic mismatch: The "--*" entry represents a pattern, but the schema has no pattern validation
2. Downstream confusion: Libraries like csstype must hardcode special-case handling for custom properties because the schema doesn't express the pattern (see frenic/csstype#189)
3. Incomplete validation: Tools consuming this schema can't programmatically validate that custom property names are structurally valid
4. Spec alignment: The CSS spec defines custom properties as a naming pattern, not individual properties—the schema should reflect this

Adding patternProperties makes the schema semantically accurate and enables automated validation of custom properties without special-case logic.

Additional details

• Pattern uses ^-- to match the CSS Custom Properties spec (CSS Variables Level 1)
• The existing "--*" entry in properties.json is preserved as documentation/reference
• Schema change is backwards compatible—only adds validation, doesn't restrict existing valid data
• Documentation updated in css/properties.md to explain the pattern validation

Related issues and pull requests

The most notable downstream consumer is frenic/csstype#63, because several large ecosystems including giants like React all depend on the csstype library, which itself depends on mdn/data. As a result, there are StackOverflow questions and GitHub issues everywhere regarding this lack of schema recognition.