Forum Moderators: open
When you say "pagerank flowing away", what you mean is "pagerank points that I could be recycling by sending to other pages of my site, I'm actually giving away to other sites". Linking out doesn't affect the pagerank of the page the links are on.
To give a simple example, a missing
</head> tag and the page would probably display just fine in most browsers - but the bot won't get the fact that the body of the page has begun, and all your precious keywords, <h1> and all the rest would be ignored. Validation ensures that your markup is not going to cause problems, and you can then concentrate on the other aspects of SEO. CSS files are rarely parsed by the bots, so there is less of a problem there (although it is still better to validate - think of your users as well!).
However, having said that, there is no need at all to link to the validation services.