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CSS Hack: The Underscore Hack

Let's start with three simple facts — as Petr Pisar found out.

  1. The underscore ("_") is allowed in CSS identifiers by the CSS2.1 Specification
  2. Browsers have to ignore unknown CSS properties
  3. MSIE 5+ for Windows ignores the "_" at the beginning of any CSS property name

Therefore, a CSS definition, e. g. _color:red is:

  1. Correct, for CSS 2.1 specification allows it (even if software validators, knowing only older version CSS 2.0, say it's a bug: they are wrong, it's correct).
  2. Ignored in any browser but WinIE
  3. Treated as color:red in WinIE

Thus, this IE's bug/feature is very simple and clear way to set CSS properties for WinIE only (MacIE doesn't have this bug/feature). It's easy to fix e. g. mis-implemented position:fixed in WinIE (see the example).

#menu {
   position: fixed;
   _position: absolute;

Same method may be used to fix the min-height property missing in WinIE (see the example 2):

#box {
   min-height: 300px;
   height: auto;
   _height: 300px;

Note: This uses another WinIE's bug, treating overflow:visible as height:auto. For details, see the article "The "min-height" Hack".

I tested it in Windows' MSIE 5, 5.5, 6, and Opera; Mac OSX's MSIE 5, Safari, and Camino; and in Mozilla and Firefox.