I hope he doesn't look at HN HTML code.
Edit to answer "how is it invalid": is this not enough? http://validator.w3.org/check?uri=http%3A%2F%2Fnews.ycombina...
taken from w3c:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
article doctype: <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">