
Most web designers define standard font families in their code but they test their sites using computers with Arial, Times New Roman and Courier New installed. I doubt many web designers test how a site looks using the Bitstream Vera Sans font, for example, that comes with most Linuxes, or the Helvetica font supplied with Mac OS X (although OS X includes most of the popular Microsoft fonts anyway).
I’m as guilty as anybody of furthering this particular injustice. The first thing I do on a fresh Linux install is copy across all the fonts from my Windows partition. This is solely so that websites look like the designers intended.