Traditional sizing allows a zero value to be specified without unit, but failing to specify a unit for the first argument of box-shadow causes the validator to fail.

The CSS declaration:

nav ul ul { box-shadow:0 3px 3px #C0C0C0; -moz-box-shadow:0 3px 3px #C0C0C0; -webkit-box-shadow:0 3px 3px #C0C0C0; display:none; position:absolute; top:38px; left:0; float:left; width:180px; z-index:99999; }

fails with the validation error '0 is not a box-shadow value' while:

nav ul ul { box-shadow:0px 3px 3px #C0C0C0; -moz-box-shadow:0px 3px 3px #C0C0C0; -webkit-box-shadow:0px 3px 3px #C0C0C0; display:none; position:absolute; top:38px; left:0; float:left; width:180px; z-index:99999; }

validates. (Other units appear to work as well.)

Original source is from https://github.com/richardshepherd/TwentyTenFive/blob/master/style.css, line 36, with the rgba() call changed, as that doesn't validate per the bug listed below:

Possibly related: http://www.w3.org/Bugs/Public/show_bug.cgi?id=11880