What does this piece of CSS do?

Question

a#logo {
    _background:transparent;
    _filter:progid:dXImageTransform.Microsoft.alphaImageLoader(src="/assets/images/header/logo.png", sizingMethod="crop");
    _cursor:pointer;
}

Answer 1

It makes Internet Explorer use PNG Alpha transparency. I also believe this is not according to the official css standard, so I would avoid when possible. However, sadly, using pure standards is a utopia in cross-browser web development...

Answer 2

It fixes an issue with IE rendering transparent PNGs by specifying a different Image Loader for IE.

It is definitely IE specific. Most of the time, though, you won't see this done in the CSS explicitly since most pages have multiple transparent PNGs on a page. Most developers use Javascript to fix this across all images on a page.

The most popular of these scripts is TwinHelix's IE PNG Fix.