90 to 95% use Internet Explorer, now, but there are still users, browsing using different versions of Netscape, AOL and others.
There are also some compatibility issues between different versions of browsers, as well as among different operating systems using the same browser.
Web site categories (ie. portals, brochure sites, online catalogues etc...)
Web site structure
File structure
Navigation structure: buttons, menus, tools, hyperlinks, forms
Frames
Scripts, style sheets, applications etc..
Buttons, menus, structure, hyperlinks, applications, forms