We do not need to check the node is not xml, its type, lowercase the name, match the rboolean regexp to remove the attribute with attr(key, null), so I propose to move the "value is null" check to the top of the attr function with immediate removeAttr call in this case.

Here is some jsPerf links:

http://jsperf.com/removeattr-new (attr with null as value)

http://jsperf.com/removeattr-set-new (attr with not null as value)

Pull request:

https://github.com/jquery/jquery/pull/1134

Thank you!