I've tweaked the (x)HTML and CSS so it gets as near to validating as it ever will. Tumblr use an iframe to insert the Tumblr iFrame Follow image and Tumblr iFrame Dashboard image, etc into the top right hand corner of the page. This throws a validation error. There's nothing I can do about that. And it's of no big concern.

The CSS is valid apart from the use of border-radius. Which is more an issue with the Jigsaw validation engine than an actual error.

All, in all, I'm quite impressed that Tumblr allows you to get syntax this good. It's a shame you can't use the HTML5 doctype with Tumblr, but again, not the end of the world.

Although almost valid, the mark-up still requires a fair bit of work. It's all a bit of a hack and could benefit from some tidying up. I'll do that in the meantime.