There are some issues with standards compliance of the new extensions.

This is because of two files: the first is
http://bits.wikimedia.org/w/extensions/UsabilityInitiative/css/vector/jquery-ui-1.7.2.css?1.7.2y

I know, this is a library, but it produces most of the errors. But everything which throws errors are things, I don't think you are using: all the browser specific border-radius-things, the "zoom" property (only available in IE) and so on.
If you don't want to change this, then please upgrade to jquery.ui 1.8. At the moment the library ONLY has browser specific border-radius for example, but not the standard compliant "border-radius" like Opera and IE9 are using it.

The second file is in your responsibility: It's http://bits.wikimedia.org/w/extensions/UsabilityInitiative/css/combined.min.css?77a

Please have a look at http://jigsaw.w3.org/css-validator/validator?uri=http%3A%2F%2Fbits.wikimedia.org%2Fw%2Fextensions%2FUsabilityInitiative%2Fcss%2Fcombined.min.css%3F77a&profile=css3&usermedium=all&warning=1

Here is the same: Why do you support insufficient implementations like -moz-border-radius but not the standard compliant "border-radius"? Wouldn't it be better to change this and support browsers, which implement this correctly but not support browsers, which only support it in a browser specific way?

Another thing is the use of "cursor:ew-resize;". Why? In the few browsers supporting it, it is exactly the same as e-resize. The CSS3 validator doesn't know it - it's still in the standard but marked as "features at risk" because it's about to be dropped. Please change this to e-resize, so all major browsers will handle it like some do today.

Version: unspecified
Severity: minor