Fonts remain a sore spot in contemporary web design. Due to licensing issues, web designers are mostly limited to a handful of web-safe fonts. Those are fonts that are commonly bundled with operating systems and browsers so that the majority of site visitors will see the site as intended. Web developers have come up with a variety of font replacement schemes, but these are inconvenient and add needless complexity to the page.
The CSS 2.1 specification includes a percentage system for font weight. If this actually worked in the browser, it would give the web designer a powerful tool for enhancing the capabilities of those web-safe fonts.
Unfortunately, most current browsers seem incapable of properly rendering CSS font weights. On the PC, IE6, IE7, Firefox, Opera, and Safari all fail. I hope this will change in the future.