HTML: Sub-pixel border

Edit: I have overseen the IE6 restriction, but I leave the answer here for others ...

Its possible with transform:scale(0.5) and put a border element with border:1px; inside. So you get a half pixel border, that (although tricked and browser dependend) is displayed on screen. But I use that trick for printing.

sure, you have to adapt the content of the element, or play with position

.outer {
  border:1px solid green;
}

.halfpix {
  -ms-transform-origin: 0 0;
  -webkit-transform-origin: 0 0;
  transform-origin: 0 0;
  -ms-transform:scale(0.5);
  -webkit-transform:scale(0.5);
  transform:scale(0.5);
  width:200px;
  height:100px;
  border:1px solid black;
  }

<div class="outer">
 
  <div class="halfpix">
    </div>
  zoom browser window if your browser does not display 
  </div>

I think you could define the width of a border using units like em which would come out to less than 1px, and it would be valid. However, like Will Martin said, for display purposes it would just round it up or down to a whole pixel.