Just read this story from Jeffrey Zeldman. According to his tests it looks like some of the big sites out there have websites that don't validate. Yet they still rank highly and are widely used. Does this mean you shouldn't care about making your website meet web standards?
Of course not! There is a big movement out there to make sites meet web standards and I am sure all these sites will be working towards improving it. The list shows that some big names are doing their best to validate. So you should too.
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