Does '!important' in CSS Affect Performance?

Is !important Bad for Performance?

It's a common practice among developers to express frustration with the "!important" rule in CSS, arguing that it disrupts the cascading nature and can lead to a cycle of "!important" overuse. However, one aspect that often goes unnoticed is whether it has any significant impact on performance.

In the realm of CSS parsing, Firefox utilizes a top-down parser that analyzes each CSS file and stores the rules within StyleSheet objects. Subsequently, it generates style context trees that encompass the final values after considering all rules in their correct order.

According to the Firefox CSS parser source code, there's a routine that explicitly handles the overwriting of CSS rules. When encountering "!important," it simply marks the rule as important without any distinguishable impact on performance.

Performance degradation, therefore, is not a substantial concern when using "!important." However, it's important to note that maintaining code readability can be compromised, as the use of "!important" can obscure the cascading nature of CSS. Thus, while it may not be detrimental to performance, its potential to harm maintainability remains a valid argument against its usage.

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