Do `!important` Declarations Negatively Affect Website Performance?

Are !important Declarations Detrimental to Performance?

While the excessive use of !important in CSS can be detrimental to code readability and maintainability, it is generally not considered a significant performance bottleneck.

How CSS Parsing Works

Most browsers use top-down parsers to process CSS. During parsing, the browser creates a StyleSheet object for each CSS file, which contains the rules defined in that file. These rules are then transformed into style context trees, which hold the computed values after applying all applicable rules in the correct order.

Impact of !important

When an !important declaration is encountered, it overrides any previously defined rules with the same selector, regardless of the importance of those rules. Browsers handle this by simply marking the affected rules as "important" during parsing.

Performance Implications

Firefox's CSS parser is optimized to efficiently handle !important declarations. It uses a simple check to determine if a rule is important, without incurring significant performance overhead. The code responsible for this check can be found in the source code:

if (aIsImportant) {
    if (!HasImportantBit(aPropID))
      changed = PR_TRUE;
    SetImportantBit(aPropID);
  } else {
    // ...
  }

Conclusion

From a performance standpoint, !important declarations do not have a noticeable impact on browser speed. However, their use can significantly harm code maintainability and readability, which should be the main consideration when contemplating their use.

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