xHTML/CSS: How to make inner div get 100% width minus another div width

The mysterious overflow: hidden; is your friend here. It stops elements adjacent to floats from extending behind the float — I think that’s the layout you’re looking for.

Here’s some slightly edited HTML: I don’t think you can have # characters in your ids:

<div id="outer">
    <div id="inner1">
        inner div 1. Some text...
    </div>
    <div id="inner2">
        inner div 2...
    </div>
</div>

And here’s the CSS to achieve the layout you want.

(I put in additional CSS for IE 6 with HTML conditional comments. I just noticed you didn’t actually need it to work in IE 6 too, but if you fancy being nice to the IE 6 users out there…)

<style type="text/css">
#outer {
    overflow: hidden;/* Makes #outer contain its floated children */
    width: 100%;

    /* Colours and borders for illustration purposes */
    border: solid 3px #666;
    background: #ddd;
}

#inner1 {
    float: left;/* Make this div as wide as its contents */

    /* Colours and borders for illustration purposes */
    border: solid 3px #c00;
    background: #fdd;
}

#inner2 {
    overflow: hidden;/* Make this div take up the rest of the horizontal space, and no more */

    /* Colours and borders for illustration purposes */
    border: solid 3px #00c;
    background: #ddf;
}
</style>

<!--[if lte IE 6]>
<style type="text/css">
#inner2 {
    zoom: 1;/* Make this div take up the rest of the horizontal space, and no more, in IE 6 */
}

#inner1 {
    margin-right: -3px;/* Fix the 3-pixel gap that the previous rule introduces. (Shit like this is why web developers hate IE 6.) */
}
</style>
<![endif]-->

Tested and working in IE 6, 7, and 8; Firefox 3.5; and Chrome 4.