Is !important bad for performance?

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.

I don't think that !important is inherently bad in terms of how quickly the browser matches rules (it does not form part of the selector, only part of the declaration)

However, as has already been stated, it will reduce the maintainability of your code, and thus likely cause it to grow unnecessarily in size due to future changes. The usage of !important would also likely reduce developer performance.

If you were being really picky, you could also say that !important adds 11 extra bytes to your CSS file, this isn't really much, but I guess if you have a fair few !importants in your stylesheet it could add up.

Just my thoughts, unfortunately I couldn't find any benchmarks on how !important could affect performance.

!important has its place. Trust me on that one. It's saved me many times and is often more useful as a short-term solution, before a longer and more elegant method to your problem can be found.

However, like most things, it's been abused, but there's no need to worry about 'performance'. I'll bet one small 1x1 GIF has more of a performance hit on a web page than !important would.

If you want to optimize your pages, there are many more !important routes to take ;) ;)

What's going on here behind the scenes is that as your CSS is being processed, the browser reads it, encounters an !important attribute, and the browser goes back to apply the styles defined by !important. This extra process might seem like a small additional step, but if you are serving up many requests then you will take a hit in performance. (Source)

Using !important in your CSS usually means developer narcissistic & selfish or lazy. Respect the devs to come...

The thinking of a developer when using !important:

1. My rocking CSS is not working... grrrr.
2. What should I do now??
3. And then !important yeah.... now it's working fine.

However its not a good approach to use !important just because we did not manage the CSS well. It creates lots of design issues -- which are worse than performance issues -- but it also forces us to use many extra lines of code since we are overriding other properties with !important and our CSS becomes cluttered with useless code. What we should do instead is first manage the CSS very well, and not let properties override one another.

We can use !important. But use it sparingly and only when there is no other way out.

I agree with you on not using it because it's bad practice, regardless of performance. On those grounds alone, I'd avoid using !important wherever possible.

But on the question of performance: No, it shouldn't be noticeable. It might have some effect, but it should be so tiny you should never notice it, nor should you worry about it.

If it is significant enough to be noticable then you've likely got bigger problems in your code than just !important. Simple use of a normal syntax element of the core languages you're using is never going to be a performance issue.

Let me answer your question with a retorical question in return; an angle that you probably didn't consider: Which browser do you mean?

Each browser obviously has its own rendering engine, with its own optimisations. So the question now becomes: what are the performance implications in each browser? Perhaps !important performs badly in one browser but really well in another? And perhaps in the next versions, it'll be the other way round?

I guess my point here is that we as web developers shouldn't think about (or need to think about) the performance implications of individual syntax constructs of the languages we're using. We should use those syntax constructs because they're the right way to achieve what we want to do not because of how they perform.

Performance questions should be asked in conjunction with the use of profilers to analyse where the pinch-points are in your system. Fix the things that are truly slowing you down first. There are almost certain to be far far bigger issues for you to fix before you get down to the level of individual CSS constructs.