Following Browser/OS versions to be covered;
Windows: IE9, FireFox (latest), Chrome (latest)
Mac:Safari,FireFox
Unix:FireFox
Detailed functional testing could be conducted on couple of browser versions (e.g. Windows IE9, Mac FireFox)
Layout, styles, page navigation, usability etc. could be covered in other os/browser versions.
IE Tester can be used if more IE versions needed to be tested.
Also HTML standards, SEO will be tested from source level to see the compatibility and usability of site (proper tags, html standards, meta tags, canonical tags, image sizes).
Also a performance test round for pages using Jemeter could be conducted to see the performance and page loading time under load.