Why Does CSS Discard Entire Rules with Invalid Selectors?

Invalid CSS Selector Error Handling: Rationale

The CSS Selectors Level 3 specification dictates that entire rules containing invalid selectors should be discarded. This decision stems from the challenges of parsing errors within selectors, ensuring consistent error handling, and maintaining forward compatibility.

Challenges of Parsing Errors

Parsing errors in CSS selectors can involve:

• Undeclared namespace prefixes
• Invalid simple selectors
• Invalid combinators or tokens

By discarding rules with invalid selectors, implementations avoid the complexity of determining the validity of partial selectors or complex selector groups.

Consistency and Compatibility

The spec ensures consistent error handling across all implementations. Invalid selectors are treated as a complete parsing failure, regardless of the recognition of individual parts. Forward compatibility is also considered, as future specifications may introduce new selector syntax that could introduce further parsing challenges.

Example: Handling ":last-child"

In the example rule involving ":last-child," the spec considers this selector invalid since it is unrecognized, even if browsers can generally identify pseudo-classes. Maintaining consistency and avoiding potential layout disruptions takes precedence.

Rationale for Discarding the Rule

Ultimately, discarding invalid rules provides the most straightforward and predictable error handling:

• It prevents implementations from having to guess at the correct behavior.
• It reduces the risk of incorrect styling or layout issues.
• It ensures consistency across different browsers and versions.

Alternative Viewpoints and Exceptions

While discarding invalid rules is generally the recommended approach, some discussions in mailing lists have suggested revisions to the specification. However, the current behavior remains unchanged due to potential web compatibility concerns, as sites rely on the existing behavior for CSS hacks.

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