On Standard Markup
One sees those little 'Valid HTML' (or XHTML or CSS) buttons, that claim the page uses standard markup around on a goodly number of sites.
I get the feeling that most people ignore them, but I tend to click on them, and in so doing test the page using the W3C's validator. I do this in no small part because I've found that the pages which display these badges nearly always fail validation.
Now, truth be told, it's usually only a couple of minor errors. My guess is that the designers tend to check carefully to eliminate all failures, add the badge, and then go on to add more content without rechecking. Mistakes happen, and though I try to keep the code of my pages strict, errors creep in now and again. Still, it seems like if one is going to put a badge on one's page to make a claim and at the same time make that claim conveniently verifiable, one would wish to make certain that the claim is, well, true.