A coworker was asking me about PNGs the other day, and I pointed him to the LibPNG page — one of the more comprehensive sites I’ve found on the PNG format. For those not aware, PNG is a lossless image format (like GIF) but it supports up to 24 bit color (like JPEG). It’s also supported by all the major browsers and many desktop applications.
Another advantage to PNG is that it supports alpha transparency. Unlike GIFs which have on-or-off transparency, PNGs support up to 256 levels of transparency. So, a properly designed PNG image with alpha transparency can look good against a background of any color.
Basic PNG functionality is supported by almost all browsers (Netscape, IE and so on), and support for PNG’s alpha transparency is also widespread. In fact, just about every browser — even the Sega Dreamcast browser — supports alpha transparency… except for IE on Windows (PNGs work there, just not the alpha transparency part).
And, as IE/Windows currently has the largest market share, it was thought that PNGs with alpha transparency essentially weren’t usable on the web. However, after re-reading the IE/Windows section, I see that there’s now a workaround to get PNG alpha transparency support in IE 5.5+!
As I understand it, IE can be induced — via some proprietary HTML — into using DirectX to render PNGs. And, as DirectX supports alpha transparency, you get alpha-channel support in IE. As all the other major browsers already have alpha channel support, it’s conceivable that PNGs with alpha channel transparency could be used throughout a site.