If I understand you properly you are a web page/site designer and want to know which browser to test your pages in.

If so, then the short answer is all of them. Clearly that is impracticable, so I would compromise and test against IE and two other, non-IE based browsers.

Why do I single out IE? Even with it's newest version it still differs in some respects from non-IE browsers in the way it displays and renders web pages.

You might also want to try 'W3C Markup validation tests' for W3C compatibility at; http://validator.w3.org/

Mark