After all the modifications I’ve been making to my website, I figured I had better make sure that it’s all still W3C valid. XHTML, yes, CSS, no.
Apparently the CSS element which I used to round the borders on tables, border-radius, isn’t included in the CSS2 specification. So apparently CSS3 is the way to go.
Unfortunately the link on the banner they give you only refers to the CSS2 specification. I can’t remember the specific website I found this link on, but a quick Google will found it: http://jigsaw.w3.org/css-validator/check/referer?profile=css3
So basically all you need is to add ?profile=css3 to the end of the normal URL they give you.