It cannot be emphasized enough that these are my own opinions, and are quite definitely not set in stone. They are, however, generally echoed by authors of books on web page design, with varying degrees of emphasis.
I had thought to attempt to classify errors, but I could not come up with a satisfactory scheme to classify them - quite often, any scheme I came up with ended up placing a common error into multiple classifications, in spite of the fact that I intended the classifications to be disjoint and comprehensive. So, I'm not going to classify them, just go through them one at a time.
I also expect that I will be digressing occasionally, and expounding on general concepts - or perhaps launching into tirades on topics that I have particularly strong feelings about. Bear with me. Don't let it provoke you into flames; try to stick with civilized discussion. As I said, this is my opinion. You're free to ignore it, and I'd quite frankly prefer that to getting involved in pyrotechnics or competitive urination.