Automatic Browser Interoperability Tester by looking at Code
===

Assuming I already have valid code, is there a tool or plugin that I can use to evaluate my website code and highlight areas where different browsers could potentially display the code differently? I am aware of websites like [browsershots][1] and I have VMs to test, but I'm wondering if there is something to shortcut the process by looking at the code and warning of well known cross-browser compatibility red flags. 

For example, when using CSS which is not universally supported yet. The following is valid CSS 2.1:

    display: run-in;

Validators which are aware of CSS 2.1 will pass it with flying colors, but really, a red flag should be thrown, indicating that it does not work in Firefox 3. On the other hand, 

    border-radius: 10px;

is actually pretty well supported in updated browsers, even though it is CSS3.

This is new in Javascript, but is also passed by validators:

    getElementsByClassName();

but it would be better if a warning popped up notifying me it won't work in IE8.

Another example is well known rendering bugs, which can be triggered by weird (valid) situations. I understand there is no replacement for opening up the website in every single browser on every system to really see how it works, but I am just wondering if there is some tool that can help check that I'm writing interoperable code as I go, so that I can test more frequently in the development process and spend less time working out bugs later. 

 [1]: http://browsershots.org/