Well, one of my students, Brendan Emery sent along this fix for PNG transparency in Internet Explorer, thus refuting my biggest con on PNG use! (honestly, I can’t think of any other cons)

[This is] a JavaScript-based PNG fix for Internet Explorer 5.5 and 6 on Windows. The fix allows IE to properly render PNG alpha transparency. [...]

Why only versions 5.5 & 6? Because Windows IE versions prior to 5.5 do not support the filter which fixes the PNG problem, and version 7 (not yet released at the time of writing in January 2006) is reported to have fixed the issue entirely.

The code can be used either by a one-time copy-and-paste or as an include file. The entire thing is completely ignored by all other browsers – in fact it is treated as an HTML comment – thanks to IE’s conditional comments. This solution is therefore light, portable and non-invasive to other browsers.