When I load jQueryUI on a Windows XP machine using Firefox 3.6.3, I get a bunch of css errors:
Error in parsing value for 'filter' Lines 18, 76, 77.
Unknown property 'border-top-left-radius' Line 274.
Unknown property 'border-top-right-radius' Line 275.
unknown property 'zoom' Lines 300,306,336,345,385,408.
Q: Should I just ignore these errors?
Yes, you should ignore these.
There are attributes in there to support all browsers, not all necessarily valid CSS, but it works.
You can see a full list of current validation errors here (jQuery UI 1.8)
Yes. jQuery UI is using properties that are not consistent across browsers. Since there really isn't any "logic" in CSS, they have to list all possibilities for all browsers.
These errors are safe to ignore.
The basic reason for these CSS errors is that every browser has its own set of CSS properties that jQuery UI makes use of. For example:
-moz-user-select is only available in Firefox and Mozilla-based browsers.
Equivalent to this property, WebKit-based browsers (like Google Chrome and Safari) have -webkit-user-select.
jQuery UI makes use of these special CSS properties available in different browsers and this gracefully degrades to nothing if the browser doesn't support that particular property. jQuery UI makes pretty sure that all its widgets appear consistent across browsers. And that is the reason why its safe to avoid such errors without any visual artifacts being generated by them.