What are conditional comments?
<!--[if IE 8]>
<p>Welcome to Internet Explorer 8.</p>
Why such comments are evil?
Usually webmasters create templates of the website for all browsers except IE, and then add IE-specific styles with conditional comments. They include separate files to improve these styles caching. It's great. But IE7- uses only 1 connection to load JS files (2 for CSS, but this doesn't change a lot), so it's additional HTTP request(s) and additional payload (in case of 1 merged global CSS file and 2 files in conditional comments there also will be some time wasted). We can prevent this.
Show me the magic!
We can detect browser on the server (by User Agent string) and just strip out conditional comments if they match current browser (IE). So comments that must work in IE are just uncommented in the document head, and latter is parsed in a common order.
All this leads to difference between "general" version of cached files and "IE" ones. So we just need to separate cached files by browser - for each browser receives its own cache file (right now 4 for IE and 1 for others), and all will be OK.
All this has been included into Web Optimizer and will be available in version 0.6.2.