jquery/jquery — comment on commit 7f6a991313380b74d5fb18782fb6b99fd6c4a22d
File:  line 0
By mikesherov:

@heygrady, no, it's calling it every time a BROWSER doesn't support pixel-only margins. If the browser fails that test, it then makes sure you're looking for a margin which is a much lighter regex. Although, you may be right that it's neglible. A perf would tell.

Aside from that though, this would need a separate support test, because pixelMargin isn't gauranteed to also do the "wrong" thing for top and left. According to CSSOM, top and left should use computed value (which is a percent if specified).