I don't get it. Does your browser not support text scaling? Firefox lets you change the font size of any webpage with CTRL+ or the View > Text Size > Increase menu and Gmail supports text scaling beautifully.
I guess this style is nice if you always want the font-size set at a particular size by default but keep in mind that every custom stylesheet and extension you add (to Firefox or any browser) incurs a performance hit.
metalmeric said about Gmail - Bigger Fonts on Aug 22 2008
beerfan said about Gmail - Bigger Fonts on Jun 14 2007
HotAZGuy said about Gmail - Bigger Fonts on Sep 22 2006