The Impact of !important on CSS Performance
Despite the concerns raised about the cascading nature of CSS, the !important declaration is not inherently detrimental to performance. As indicated in Firefox's CSS parser code, the handling of the !important flag seems to be a straightforward check.
The parser creates a StyleSheet object for each CSS file, and each object contains CSS rules. The parser then constructs style context trees that hold the end values after applying all rules in the correct sequence.
In this process, the parser can quickly identify rules impacted by !important without incurring significant overhead. This suggests that !important has minimal effects on performance in terms of rendering speed.
However, it's essential to note that maintaining CSS can become difficult when !important is used excessively. This complexity, rather than performance concerns, may be the primary reason to avoid using !important.
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