Pages sit where they should on the screen
This one came from putting Floati next to the reference browser on ordinary reading sites and, whenever something looked off, asking both browsers where they had actually put the box. Six faults turned up, all of them in the plumbing that decides how wide something is and where it goes. The clearest example: A Linux news site was drawing its entire article column off the left edge of the window, with the text cut in half, because the site asks for a negative amount of inner spacing and every browser is supposed to treat that as none. There were five more like it, each small on its own and each capable of wrecking a page. Every fix was checked by rendering the same markup in the reference browser and comparing the result down to the pixel before it was allowed in.