CSS3 Color says the hue in e.g. hsl(120.0, 100%, 50%) is parsed as a <number>, which I think means it can be a non-integer. Mozilla does accept that colour, but WebKit doesn't.
Verified difference between Firefox 2.0.0.5 (shows green background) and a local debug build of WebKit r24792 with Safari 3 Public Beta v. 3.0.2 (522.12) on Mac OS X 10.4.10 (8R218). Note that Opera 9.21 does not show a green background, either.
Opera doesn't support HSL yet.
This is a very easy fix. Just righting up a new layout test for this and I should have a patch up in a little bit.
Created attachment 15760 [details] patch
Comment on attachment 15760 [details] patch r=me
<rdar://problem/5373922>
Landed in r24793.