Nowadays almost every browser has DevTools, which makes web testing way more easier. But, unfortunately not all testers are using it: some of them just don't know that DevTool exists, some of them don't know capabilities of DevTools, some of them don't know how to use it.
So I want to recommend a great CodeSchool tutorial about Chrome DevTools. It is very clear and simple and you can not just learn theory there, but do some real excercises. The great part is that DevTools of different browsers are very similar (especially Chrome and FireFox), so if you learn to use one you can easily use the other too.
Also I want to recommend a video Secrets of the Chrome Developer Tools from Oredev Conference, where Patrick Dubroy from Google Chromium team talks about some advanced staff that you can do with Chrome DevTool.