IMO:
While W3C covers a wide range of contexts, I think it is fair to say their focus is web pages displayed in browsers. If you are trying to produce aesthetically pleasing layouts that are flexible across devices, browsers, window geometries and user font sizing then em and % can work well for fonts and related boxes. In mobile web apps targeting a single device, many of these factors are fixed. In addition the developer often doesn't want the user just scrolling down to overcome any deficiencies and may disable zoom. By using absolute measurements, I can tightly control the layout down to a couple of pixels and ensure everything looks okay while using the full screen and no more. Even it isn`t perfect due to descenders and the like but it is much better than leaving it up to the browser designer to decide how big the base font should be. The "right answer" is probably a careful combination of the two. Use px when you really do care about the size from a placement/intersection perspective. Use % or em when you can accurately express what you want relative to something else.
