Check the Easy Stuff First!
While the difference you see between browsers might be due to some obscure browser bug or some new technique you're using, nine times out of ten, it's just something simple. It's easy to blame a problem on a new technique and spend hours debugging it, only to find that you're changing one file but uploading and viewing a different one.
The general easy stuff:
Make sure you've uploaded the actual file you want to test (see page 353).
Make sure you've uploaded the file in the location that you think you have.
Make sure you've typed the URL that corresponds to the file you want to test.
Make sure you've saved the fileincluding the very latest changesbefore you upload it.
Make sure you've uploaded any auxiliary filesCSS, images, music, videos, etc.
Make sure the upper- and lowercase letters in your URL match the upper- and lowercase letters in your filenames exactly. And make sure you haven't used spaces or other punctuation in filenames at all.
Make sure the problem is not the browser's fault. The easiest way to do that is to test it in another browser.
On the following three pages, I'll give you some ideas of how to check the easy stuff in HTML, XHTML, and CSS.