Why Does Invalid CSS Selectors Cause Entire Rules to Be Dropped?

Invalid CSS Selector Causes Rule to Be Dropped: A Deeper Rationale

The CSS Selectors Level 3 specification directs user agents to discard rules containing invalid selectors. This includes unrecognized tokens, combinators, and namespace prefixes. The purpose of this handling is to ensure consistency, forward compatibility, and prevent layout disruption.

Arguments for Dropping the Entire Rule

• Ambiguity in Selector Parsing: Identifying the boundaries of a selector, selector list, and declaration block can be complex, especially with unrecognized selectors or functional pseudo-classes that accept selector lists. Guessing the boundaries risks introducing errors in layout.
• Unknown Pseudo-Classes: Implementations may not necessarily recognize all pseudo-classes, leading to confusion in parsing. By discarding the entire rule, it simplifies error handling.
• Consistency in Error Handling: Treating unrecognized selectors the same as malformed selectors ensures predictable behavior and avoids potential confusion.

Alternative Approaches

While discarding the entire rule makes sense in most cases, it can be frustrating when only a small part of a selector is invalid. Discussions have been held on the www-style mailing list about adopting a media query-style invalidation approach, where only the invalid part is discarded.

However, this approach raised concerns about web compatibility. Many websites rely on browser behavior of discarding rules with unrecognized selectors, such as browser-specific hacks or selector filtering. Changing this behavior could break existing sites.

Conclusion

Given the complexities of selector parsing and the need for consistency, the decision remains that invalid selectors will cause the entire rule to be dropped in CSS. This ensures predictability, forward compatibility, and avoids potential layout issues.

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