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If I would be at your places I would have played natural. Instead of adding keywords to all the images, I would have randomly added Alt tags to images and tags would define images and not keywords! I will recommend using the same idea when building content. Just do not consider about keyword percentage in the document and make it as useful for reader as possible and you will see rankings as well as conversion ratio will increase.

Google is a different case being run through the validator. I actually read an article on why google's site do not validate. The reason is that they send so much traffic, it actually saves them a good amount of money not closing tags that do not matter. Things like adding a self closing / to an img tag and the sorts. While I do not think that validation is a ranking factor, I wouldn't totally dismiss it. It make code easier to maintain, and it has actually gotten me jobs before. Clients have actually ran my site through a validator before and hired me. Plus funny little things work out too, someone tested my site on nibbler and it came back as one of the top 25 sites. I get a few hundred hits a day from it. I will take traffic any where I can get it.