Personally, I think the worst thing one can do with HTML is neither to mix it with other languages, nor blink, nor anything to do with links or frames. No, the worst thing you can do is to make your text unreadable, because that is not just inconvenient, it defeats the very reason for which most webpages exist.
Some of the most common idiocies:
- Background images with strong, fine-grained contrast. No matter what color the text, it will be hard to read.
- Tiny, absolute fontsizes.
- Text of medium brightness on a black background; really bad if your monitor isn't perfectly sharp.