There is no frickin way I will install every single OS with every single browser of every single version.
To check if my website is compatible with "everybody".

I know, you can't make it compatible for everybody, since there is hell-lotta crap between screen and server.
But how to check with which "combinations" does website use 100% of it's functionality to have "wide-compatibility", like does?

I also mind such old craps like msie6.0 and winnt4.0


You really don't have to worry about the OS so much as the browsers and the language that the page is written in. I would say keep it simple and use applications that depend on the server not the client like html, php, css and mysql. PHP converts to html which most browserrs have no trouble with. It is when you try to get fancy with active-x, java, or net-framework and other client based scripts that you run into issues.

commented: Helpful tracks. But I still have to use such. +2

I mean, things that other browsers do not cover. I am using only HTML, JS and CSS. Some things are supported by one browser and not another. Meanwhile msie is so bad at this, I want to format my drive on which it is located.