zoom: 1 and hasLayout

A meaningless property set purely to switch on an undocumented rendering mode.

In 2026: Harmless now: zoom is a real property again and 1 means no change. In IE6 and 7 an element either had layout or did not, which decided whether it contained floats, honoured width, or drew backgrounds at all. Nothing in CSS turned it on, so people used any property that happened to.

Where it came from: Reverse engineered by the CSS community, roughly 2004. Microsoft finally documented hasLayout on MSDN in 2005. Wikipedia ↗

<style>
/* Any one of these flipped hasLayout on in IE6 and IE7 */
.fix { zoom: 1; }          /* the usual choice: no visible effect elsewhere */
.fix { height: 1%; }       /* the other usual choice */
.fix { display: inline-block; }
.fix { position: absolute; }
.fix { float: left; }
</style>

<div class="fix">
  Without one of these, IE6 would refuse to wrap this box around its floats,
  drop the background, or ignore the width outright.
</div>