They can cause Internet Explorer to ignore the markup between comments or to include part of a comment as if it was regular markup. Conditional comments are specially constructed HTML comments that Internet Explorer on Windows may treat differently from other browsers, optionally based on IE's version number. Luckily, as of version 5, it deliberately supports a rather safe-to-use hack called “conditional comments”. Due to its relatively poor level of standards support, Internet Explorer tends to be the subject of most CSS hacks.
