How to testTesting css + html

For Javascript, PHP or any other language I know it's easy to check that your code will generate an expected output.

What I'm trying to learn is how companies like Facebook, Google etc. test their CSS and HTML. How do they test if the layout is ok and that a certain div won't overflow when a string is too large for example? Do they just trust that the new guy that's doing that work did it correctly? There has to be some kind of testing. Is it his team leader who will proof read all his HTML and CSS? Or do they use some methodology or framework that will reduce the number of errors on the final output?

Hope I was clear enough.

Thank you for your help.