All the parts about avoiding px units for width, line-height, etc. are somewhat outdated with modern browsers. The default is now to "zoom" the pixel dimensions as well.
It's also much more practical to just use pixels. With relative units, nested elements that adjust the font size will quickly destroy any rhythm you were going for, unless you deliberately write extra CSS rules accounting for every possible nesting, with a bunch of annoying math involved.
Furthermore, if you're styling a web application rather than a document, relative units are just not going to work out. Most UI elements are of fixed dimensions, with fluid areas being the exception. Within a rigid UI, defining your type in px units is a preferable approach.
It's also much more practical to just use pixels. With relative units, nested elements that adjust the font size will quickly destroy any rhythm you were going for, unless you deliberately write extra CSS rules accounting for every possible nesting, with a bunch of annoying math involved.
Furthermore, if you're styling a web application rather than a document, relative units are just not going to work out. Most UI elements are of fixed dimensions, with fluid areas being the exception. Within a rigid UI, defining your type in px units is a preferable approach.