In an idle moment, I had a look at the stylesheet. I'm not a web styling expert, but I need to sort out the stylesheets for theIntelligentBook.com. Interestingly, the BBC's font size and line height are specified in pixels – years ago that was said to be a no-no because 13px might not be so readable on a denser display or a handheld. But trying to use "em" measurements or anything like that was horribly incompatible between browsers. These days, Safari and Firefox still happily resize text smaller or larger when asked (Ctrl-+), even if it is specified in pixels. So I wonder if the "old pixel" has become the new standard resolution-independent measurement: "how big would this have looked on an old SVGA monitor"?
EDITED: Oops, as bods commented, it is mostly em-based after all (but with occasional stray px font-sizes for parts like ".storyextra h2" that I happened to hit in my initial idle look)