Something similar to http://my.opera.com/dragonfly/blog/style-profiler-preview from Opera or https://bugs.webkit.org/show_bug.cgi?id=74004 from WebKit.
AFAIK, there's currently no way to see how much time is spent on evaluating certain CSS rule, and how much time is spent on document reflow and/or repaint.
Created attachment 584412 [details]
Screenshot of WebKit's CSS selctor profiler
A generic profiler would be great, but a CSS selector profiler would also be very useful.
This needs a feature page.
kdangoor, would you like to do the honors or should I?
I don't believe anyone uses feature pages in Mozilla any longer. I suppose mockups, user stories and the like are still needed, but that's a task for whoever starts working on this issue.
Created attachment 8404807 [details]
Screenshot of Opera's old Style Profiler
Opera's old style profiler
I'm going to work on this in bug 975522.
*** This bug has been marked as a duplicate of bug 975522 ***
Is bug 975522 really a duplicate of this? That one seems to just cover "was a selector used or not", while this is about time spent per selector. But maybe you plan to add more than what the description and existing patch over there conveys...? :)
(In reply to J. Ryan Stinnett [:jryans] from comment #7)
> Is bug 975522 really a duplicate of this? That one seems to just cover "was
> a selector used or not", while this is about time spent per selector. But
> maybe you plan to add more than what the description and existing patch over
> there conveys...? :)
No, thanks - it's not a dupe of this. I'm fairly sure, bug 834865 is what I was thinking of.