I doubt OOUI will use the same classes, but being styled identically would be nice. I'm working on a way to generate OOUI compatible output from PHP, so that the same classes actually can be used, and ideally the server-side widgets I'm working on could be used as the way MWUI generates widgets.
It's important to identify that the core principal behind OOUI's design is abstraction, so widgets in OOUI will not, and need not, have semantic HTML output which can be styled using generic CSS targeted at such. It turns out that even in modern browsers, using HTML5 and CSS3, that it is not possible to achieve cross browser compatible styling on many semantic elements that have been around since the dark ages, like fieldset, legend, button, or input. OOUI also uses abstraction for events for similar reasons - but I digress. The point is, we really look forward to having output of certain (simple) OOUI compatible widgets from PHP, because it supports out approach to abstraction, even in a non-JS environment.
I'm on vacation until the 13th, so I might not check back until then. When I return, I will be working again on my PHP library, and we should talk about how we can eventually merge all this stuff together.
- Trevor