Why Does CSS Discard Entire Rules Containing Invalid Selectors?

Invalid CSS Selector Causes Rule to be Dropped: Understanding the Rationale

In the CSS Selectors Level 3 specification, user agents are instructed to handle parsing errors by observing certain rules: invalid simple selectors, combinators, or tokens necessitate discarding the selector and possibly the entire group of selectors it belongs to. This specific handling raises the question:

Why does the specification dictate discarding the entire rule, rather than simply removing the unrecognized selector and retaining the rest of the declaration block?

The primary reason is to ensure consistency in error handling and to avoid the potential for erroneous interpretations. Determining "the rest of the rule" or "the rest of the selector list" based on the presence of unrecognized selectors could be challenging for implementations. Attempts to guess could lead to incorrect handling, layout issues, or inconsistencies across browsers.

Additionally, this approach aligns with the forward compatibility principle in CSS specifications. As the industry evolves and new pseudo-classes and functional notations are introduced, it becomes crucial to define how unrecognized selectors are handled. By establishing clear error handling rules, the spec ensures uniformity and prevents potential ambiguities in the future.

Furthermore, eradicating the entire rule eliminates the risk of applying potentially undesirable or unexpected styles to elements due to incomplete or incorrect parsing. Discarding the rule altogether guarantees that the layout of the page remains intact and consistent across browsers.

It is worth noting that some layout engines may have different approaches to handling unrecognized selectors, such as ignoring prefixed selectors or discarding only unrecognized portions while applying the rest. However, these behaviors may vary depending on the implementation, making it essential for developers to rely on the spec-defined behavior to ensure cross-browser compatibility.

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