png.js attempts to replace GIF images that have a class name of png (ie. <img src="image.gif" alt="" class="png" />) with PNG images where supported; in the case of IE:win32, the necessary filters are applied which enable alpha transparency (a big PNG feature) to work. Disappointing on MS' part that it isn't supported natively, but oh well.
PNG alpha transparency only supported under IE 5.5:win32 and up.
Image width and height must be specified under IE for PNG to work.
PNGs as background images may break under Netscape 6.x.
PNG backgrounds will break under IE:mac. (eg. on a DIV element - known browser bug.)
PNG backgrounds do not tile under IE:win32.
"none.gif" placeholder image required for IE:win32 (due to proprietary filter crap.)