I think Rundle would appreciate this list more than me, but I can definitely go along with it.
- When your markup is readable by humans
- When your functionality degrades gracefully
- When you can increase the text size willy nilly and it doesn’t break your layout
- When your site works in Internet Explorer without you trying
- When your site’s hierarchy is hierarchical even without your stylesheet
- When your url structure makes sense
- When you make a really awesome form
- When the client says “this is great, this is exactly what we wanted” and you agree