Getting these PNGs to actually be transparent is no longer a major issue—we’ve used a few different approaches over the years, but settled on Angus Turnbull’s iepngfix.htc as the most straightforward drop-in solution out there. The way it works is just really smart.

But while building out some recent client projects, we just couldn’t get it to catch. We checked and double-checked the file paths, and re-exported the PNG images, and finally copied a working example from a live client site, but nothing seemed to do it. Because IE has no decent debugger, I removed the conditional comment to see what I could learn in Firebug.

It turns out that our staging server sends the .htc file as text/plain, while the working client version was serving text/x-component. Changing out server config to mimic this fixed the problem. Later, some Googling revealed that the release notes for the newest beta version address this (and even include a PHP solution that serves the file while sending the correct headers).

There are still a whole host of limitations: no repeating images, weird layering glitches that happen even in IE7, and really bizarre link clickability issues. But iepngfix gets far enough to solve 90% of the problem.