[Coda]

Technically that's working as intended. Those fonts don't have a boldface variant, so it's falling back to the closest match it has -- that is, the normal weight. This is consistent with the HTML5 specification, which says that just because you can specify different boldness levels doesn't mean every possible level will look distinct.

Safari is inventing a fake boldface variation of the font on the fly. The quality of the letterforms isn't going to be as good as a proper boldface font would have had.

Italics often work the same way -- a font that doesn't have an italic variant gets smooshed diagonally to create an artificial oblique form.

EDIT: I gotta say, I've never thought of Safari as being "too clean," especially not compared to Chrome. o.o