Step 3c: CSS selectors, specificity, and cascade

[thomasnemer]

Parent: #22
Prerequisites: Step 3b

Goal

Implement selector parsing, matching, specificity calculation, and the cascade algorithm.

Implementation

• Selector struct: compounds + combinators (descendant, child, sibling)
• CompoundSelector: type, id, classes, attributes, pseudo-classes
• Attribute selectors: [attr], [attr=val], [attr~=val], etc.
• Pseudo-classes: :hover, :focus, :first-child, :last-child, :nth-child(an+b), :not()
• Specificity(a, b, c) calculation and comparison
• Cascade: collect matching declarations, sort by origin/specificity/order
• Origin: UserAgent, Author, with !important handling

Tests

• Selector parsing, specificity calculation, cascade ordering
• ~15 tests

Acceptance Criteria

• cargo test -p ie-css passes
• Clippy clean

[thomasnemer]

Context from #68 implementation

Parser API

• parse_stylesheet(css) -> Stylesheet with Stylesheet { rules: Vec<Rule> }
• parse_declarations(css) -> Vec<Declaration> for inline styles
• Rule { selectors: Vec<String>, declarations: Vec<Declaration> } — selectors are RAW STRINGS, not parsed yet
• Declaration { property: PropertyId, value: CssValue, important: bool }

Value types (values.rs)

• PropertyId enum with ~55 CSS properties
• CssValue: Keyword, Length(f64, LengthUnit), Percentage, Number, Color(CssColor), String, Auto, Inherit, Initial, None
• CssColor { r, g, b, a } with hex/named/rgb() parsing
• LengthUnit: Px, Em, Rem, Percent, Vw, Vh, Vmin, Vmax, Ch, Ex, Pt
• is_inherited(PropertyId) -> bool, initial_value(PropertyId) -> CssValue
• parse_property_id(name) -> Option<PropertyId>

For selectors (#69)

The Rule.selectors field currently stores raw selector strings (e.g., "div > .class#id"). The selector module needs to:

1. Parse these strings into structured Selector types
2. Implement selector matching against DOM nodes
3. Calculate specificity
4. Implement the cascade algorithm that sorts matched declarations

The existing selector.rs has basic types (SimpleSelector with tag/id/classes) that should be replaced with the full compound selector model with combinators.