That is a little strange: http://jsfiddle.net/timmywil/j29XG/5/

After reported it, I investigated this bug a little bit. My guess is that in IE 7,8 there is no "getElementsByClassName" function, so jquery is looking for all elements in document by using "getElementsByTagName('*')" and test each element against for the class name. When I add my customized selector, it check every elements for the class name and for my selector instead of applied my customized selector after filtering elements by class name.

The jQuery team should investigate if this bug occurs also for jquery selectors such as ":visible", ":hidden", ... It would slow things down for browsers who doesn't have the "getElementsByClassName" function.

Investigate and then do what? The alternatives are terrifying.

Do what? Applied selectors after filtering elements by class name.

Check this: http://jsfiddle.net/j29XG/9/

It apply ":mySelector" on all document elements (meta, script, div, ...) because of ".className:mySelector" and apply ":mySelector" again on elements returned by ".className:mySelector".

So, I think there is a problem. But if you are too lazy to investigate if this bug apply also on jquery selectors and to fix this bug, fine, just forget this bug report.

This bug also apply for jquery selectors such as ":visible", ...

http://jsfiddle.net/mRYn9/

By fixing this bug, jQuery selectors will be faster on browsers which doesn't support "getElementsByClassName" function.

A other improvement for "getElementsByClassName" jquery fallback would be:

var arrElems=document.body.getElementsByTagName('*');

arrElems.push(document.body);

instead of: var arrElems=document.getElementsByTagName('*');

We are looking for elements with class attribute, we doesn't need to catch useless elements such as "comments", "meta", "script", "style",...

This would only be changed by adding another function call to the stack, which most of the time would be a bigger performance cost than extra loops.