- No alt or longdesc attributes on images.Very bad - I couldn't post on some bulletin boards with images turned off because the "reply" image (the one that brings up the actual form to reply from) had no alt attribute
- Using any kind of font tag. Style sheets forever!
- Not conforming to W3C standards without a valid reason (which would be "I want to make it compatible with Browser X.x")
- Knowing that your code isn't viewable in any browser, but not doing anything about it! Tip: graceful degradation is good.
- Writing bad HTML code - it just isn't on, especially when there are a load of applications like HTML Tidy to help you clean up your code.
The things and more above might lead to your page
not being accessable
to people with disabilities - or perhaps to people who can't read that garish yellow text on a black background.