Well, no, it wouldn't work for proportional fonts, because spacing is so dependent on the metrics of the particular font used, including rules regarding the kerning of characters. And just because you're using (and specifying) Arial, doesn't mean that everyone looking at your page is using it as well; they might not have an Arial font on their systems. Indeed, in a text-based browser everything is monospaced anyway.
There are approaches to right/left aligning text, but none apply to the contents of select entities.
