Does `!important` Really Hurt Performance?

Does !important Damage Performance?

The controversial CSS exclamation mark (!) and important keyword have raised concerns about their impact on performance. Despite their apparent violation of the cascading nature of CSS, it's worth delving into whether they truly compromise performance.

To answer this question, we examine Firefox's CSS parser code to uncover how !important declarations are handled.

Firefox's parser checks the "important" flag for each rule. If a rule is marked as !important, it sets the relevant flag and continues parsing subsequent rules. This process is relatively straightforward, suggesting minimal performance impact.

Additional evidence suggests that Firefox's Style Context Trees (SCTs) capture the end values of CSS rules, including those with !important declarations. SCTs allow the browser to efficiently resolve conflicts between multiple rules by prioritizing those with the highest specificity or !important status.

Therefore, based on the analyzed code and architectural details, !important declarations do not significantly impact performance in Firefox.

While !important may not directly affect performance, it raises concerns about maintainability. Misuse of !important can lead to a reliance on a cascade of higher specificity or importance values, making CSS stylesheets difficult to manage and troubleshoot. Therefore, it's recommended to use !important judiciously to avoid unnecessary complexity.

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