CSS Top margin before out of nowhere

A typical CSS problem but I fall for it again and again. The whole site is shifted down 10-20 pixels in spite of every imaginable variant of margin:0 in the <html> and <body> elements.

So what was it? A <h1> somewhere in the floating layout whose margin-top pushed everything down.

Note to self: Always check for headings!

Useful link: Margin Collapsing