Step 3b: CSS parser — rules, declarations, and value types

[thomasnemer]

Parent: #22
Prerequisites: Step 3a

Goal

Parse CSS into structured rules with selectors and declarations. Define CSS value types and ~40 properties.

Implementation

• Parse stylesheet into Stylesheet { rules }
• Parse qualified rules: selector + declaration block
• Parse declarations: property: value pairs with !important
• Shorthand expansion: margin, padding, border, background, font, flex
• Value types: Length (px/em/rem/%), Color (#hex, rgb(), named), keywords
• PropertyId enum for all ~40 properties
• Property inheritance classification
• Initial values per property
• calc() basic support

Tests

• Parse simple/complex rules, shorthands, !important
• Value parsing: colors, lengths, keywords
• ~15 tests

Acceptance Criteria

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

[thomasnemer]

Context from #67 implementation

CSS Tokenizer API (tokenizer.rs)

• CssTokenizer::new(input) -> impl Iterator<Item = CssToken>
• CssToken enum with 24 variants: Ident, Function, AtKeyword, Hash(String, HashType), String, Url, Delim, Number(f64, NumType), Percentage, Dimension(f64, String), Whitespace, Colon, Semicolon, Comma, brackets, Cdo, Cdc, Eof
• HashType: Id vs Unrestricted
• NumType: Integer vs Number
• Input preprocessing: CR/LF normalization, null → U+FFFD
• Comments consumed and discarded (not emitted)
• Escape handling: \ + 1-6 hex digits, \ + literal char

For the CSS parser (#68)

The parser should consume CssToken values from the tokenizer. Key token patterns:

• Rule: Ident/Hash/Delim(.)/Colon/... (selector) CurlyBracketOpen (declarations) CurlyBracketClose
• Declaration: Ident Colon (value tokens) Semicolon
• At-rule: AtKeyword (prelude tokens) CurlyBracketOpen/Semicolon
• Value: Ident/Number/Dimension/Percentage/Hash/Function/String