lucy24 - 11:13 pm on Mar 25, 2012 (gmt 0)
Right. The html validator doesn't look at the contents of an attribute, it only checks that it's present. An empty alt is appropriate when the image is purely decorative. Otherwise you can often use a generic "see text" or "see caption" so the image-less reader knows you didn't simply flake out on them ;)
The css validator goes one step further by checking whether a property uses a valid system of naming or measurement. For example you can't say "width: green" or "background-color: 100em".