The following three lines are the same length at all text sizes on my Safari and Firefox and Chrome. I don't know what happens on the Window versions or on IE right at this moment. But I will when I stick this on a server and fire up my VirtualBox Windows! At my normal text size, all three are sharply defined, the second, a pic sized in ems gets progressively more blurred on enlargement.
ABCDEabcde012345 ... html text
... pic sized in ems
... pic native px size.