blooblog

The Underscore Hack relies on Internet Explorer (5 and up) ignoring underscores at the start of a property name. To illustrate, IE treats _position the same as position, enabling authors to write rules only it can understand. This is especially useful in certain cases where IE's CSS implementation is broken.

This method will prevent the CSS from validating as pure, although it is allowed under CSS 2.1's syntax and grammar as a vendor-specific extension.