!important 对性能有害吗? [英] Is !important bad for performance?

问题描述

我讨厌它们，它违背了 CSS 的级联特性，如果你不小心使用它们，你最终会陷入添加更多 !important 的循环中.

但我想知道它们对性能有害吗?

编辑
从(快速)回复中，我可以得出结论，它不会对性能产生(显着)影响.但很高兴知道，即使这只是作为劝阻他人的额外理由；)

编辑 2
BoltClock 指出，如果有 2 个 !important 声明，规范说它会选择最具体的一个.

解决方案

它不应该对性能产生任何明显的影响.在

现在，您可以很容易地看到，在上述对象模型的情况下，解析器可以轻松标记受 !important 影响的规则，而不会产生太多后续成本.性能下降不是反对 !important 的好理由.

但是，可维护性确实受到了打击(正如其他答案所提到的)，这可能是您反对它们的唯一论据.

I hate them, it defies the cascading nature of CSS, and if you don't use them with care you end up in a loop of adding more !important.

But I want to know are they bad for performance?

EDIT
From the (fast) replies I can conclude it won't have a (significant) impact on performance. But it's nice to know, even if it's just as an extra argument for discouraging others ;).

EDIT 2
BoltClock pointed out that if there are 2 !important declarations the specs says it will pick the most specific one.

解决方案

It shouldn't have any discernible effects on performance. Seeing Firefox's CSS parser at /source/layout/style/nsCSSDataBlock.cpp#572 and I think that is the relevant routine, handling overwriting of CSS rules.

It just seems to be a simple check for "important".

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

Also, comments at source/layout/style/nsCSSDataBlock.h#219

    /**
     * Transfer the state for |aPropID| (which may be a shorthand)
     * from |aFromBlock| to this block.  The property being transferred
     * is !important if |aIsImportant| is true, and should replace an
     * existing !important property regardless of its own importance
     * if |aOverrideImportant| is true.
     * 
     * ...
     */

---

1. Firefox uses a top down parser written manually. In both cases each CSS file is parsed into a StyleSheet object, each object contains CSS rules.

2. Firefox then creates style context trees which contain the end values (after applying all rules in the right order)

From: http://taligarsiel.com/Projects/howbrowserswork1.htm#CSS_parsing

Now, you can easily see, in such as case with the Object Model described above, the parser can mark the rules affected by the !important easily, without much of a subsequent cost. Performance degradation is not a good argument against !important.

However, maintainability does take a hit (as other answers mentioned), which might be your only argument against them.

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