Error: Irreparable invalid markup!!!

I lack a better word to replace the strikethrough, anybody? I’m also finally getting annoyed at the way that my Mac decides to display this page. While everything is fine and dandy on good ol’ Winbloze, things look slightly less pretty (in a purely layout-ish sense, everything else is truly pretty) on the Mac. I will now dedicate several hours to poking around and figuring out how to make stuff look just as good on the Mac. For example, the trusty </center> does not break the existing <center> tag, instead it appers that my entire page is now centred with the exception of little dangly bits here and there.