A lot of the W3C specs are actually so poorly specified that none of the browsers can even figure out how to implement them. E.g. in CSS the width of a pixel is defined relative to the length of each user's arm.
I had to see this for myself to believe it.
"The reference pixel is the visual angle of one pixel on a device with a pixel density of 96dpi and a distance from the reader of an arm's length. For a nominal arm's length of 28 inches, the visual angle is therefore about 0.0213 degrees. For reading at arm's length, 1px thus corresponds to about 0.26 mm (1/96 inch)."
Once upon a time the W3C site pointed out that they did not define standards. They offered recommendations with the idea that people would implement them, see what worked (or didn't), and at some point a standards body (ISO, ECMA) would issue the actual standard.
However, so many people keep referring to W3C recommendations as standards (mostly as a way to brow-beat people into following one or another preferred practice) that the W3C changed their tune and decided to go along for the ride.
Instead of seeing W3C TRs as the last word we would be better served if people viewed them as (likely) incomplete works-in-progress on the way to ironing out well-defined standards.
It's easy to snidely dismiss that idea, but seriously, how would you define a unit of length in an environment that is supposed to translate between multiple devices and media types? A "pixel" makes little sense when drawing on a sheet of paper or a super-high DPI screen where pixels aren't individually visible. It can't be a fixed real-world distance, because pixels would sensibly be larger at movie-screen distance than at arms' length. They basically defined it in terms of the arc-length of the picture upon your eye, which is completely reasonable to me.