That's not how it works in reality though. Every browser ever made has bugs, sites will have to work around those bugs, and depending on the developers in question you might get one, two, or as many browsers as the developer is willing to test in, sets of workarounds.
Not to mention so much of the web is still a horrible mess of tag-soup where each browser vendor has had to make up support, and each other vendor has had to guess or reverse engineer what its competitors are doing.
Further to that Opera has, on at least two occasions that I remember, encountered examples of broken code because they support too many standards. Most notably in the case of when they introduced WebForms 2 support and several sites using previously non-standard values (the spec said all unknown values should be treated as "text") for <input type=""> suddenly were using values that meant very different things. There were also many problems caused because Opera was I believe the only browser to ever correctly support the third parameter of addEventListener(). Opera has since had to break its support because Mozilla concluded that too many sites would break if they implemented fully-correct support and ultimately I believe the spec has, or will be superceded with a version that reflects that end result.
And as a final point in my not-so-subtle suggestion that the biggest problem with site compatibility is actually web developers not knowing their job properly: Opera 10's user-agent is actually "Opera/9.80 (Windows NT 5.1; U; en) Presto/2.2.15 Version/10.00" because several high-profile sites assumed that no browser would ever reach a double digit version number (Flash has also seen this problem in a few places since version 10 of the plugin arrived.)