What's happening is that the <h3> tags are all nested, since they don't auto-close in this situation, and modern browsers define the <h3>'s size as "font-size: 1.17em;" which of course adds up over time. I'm not sure how IE5 interprets the page. Maybe I can get an older embedded version of Firebug to check.
I think it's important to not break even older websites even if they have totally busted HTML, but this is probably such a rare case that no one ever thought of it, and at this point it'd probably be much more disruptive to try and "fix" it.