Are there any developers that offer themes that have a defined list of browsers tested, on desktop and mobile devices, that are also periodically updated to account for new browser versions?
New browser versions not only follow standards more closely but are very well coded for rendering almost anything. There are no real forward compatibility issues when it comes to styling (outside bugs).
Modern browsers can render garbage code as though it was written by Tim Berners-Lee. If a template works in IE6 and IE7 you can rest assured it will render in any versions above that since the vast majority of problems are related to Microsoft.
If you have specific needs (mobile, email, accessibility) there are various testing applications for rendering sites.
Okay, well, I've found http://themeforest.net/category/wordpress
Details on each theme include: Compatible browsers WordPress version Documentation detail Layout type Types of source files included Widget ready
Looks pretty good to me. Will give it a try.