http://trac.webkit.org/changeset/219050/webkit removes Yosemite support
http://trac.webkit.org/changeset/212211 removes a lot of checking for unimplemented Objective-C methods
Snow leopard compatible version?
The latest version is available here: https://sourceforge.net/projects/leopard-webkit/files/537/%28Snow%29%20Leopard/WebKit-537.78.2_4-%28Snow%29Leopard.dmg/download It's more or less equal to what shipped with Safari 7 (OS X 10.9). Follow the instructions to install Safari 5.0.5 as it won't work with Safari 5.1 .
Well, an Intel build would be easliy done - but a 10.6.8 build needs quite some work to get it working right. In order to have it build and work on 10.5.8 one has to disable/workaround/reimplement things that require OS support relying on feature that were introduced in more recent OS versions only. Obviously for 10.6.8 many of the same things that apply to 10.5.8 also apply but others don't and again others would have to be solved differently. After all WebKit, contrary to Firefox, integrates into...