It always seemed like it would be interesting to serve a gif image to Internet Explorer 6 and a png to other browsers, instead of using a CSS behavior file (like IE PNG Fix or SuperSleight). So now that IE 6 is nearly gone, here’s how.
The gif is surrounded with a conditional comment.
<img class="modern" src="logo.png" /> <!--[if lt IE 7 ]> <img src="logo.gif" > <![endif]-->
At this point all browsers display only the png, except IE 6 displays both the png and the gif.
So in the css, to keep the png from displaying to IE 6:
img.modern {
_display:none; /* IE 6 */
}
The underscore hack before display: none targets IE 6 only.
OK, so this uses a conditional comment and a CSS hack; a png fix or background image is probably a better choice. But since IE 9 works only in Windows 7, the IE hack or conditional comments and style sheets for IE 7 and IE 8 are going to be with us for some time.
More information on hacks and conditional comments can be found on Web Designer Wall and Paul Irish.