How could I select IE7 with pure (valid) CSS?
If you don't want to use a conditional comment (outside the CSS, e.g. defining a separare <style>
section), the only thing you can use is CSS Hacks. See here for a "IE7 only" hack.
IE does support conditional comments, an IE-specific HTML comment syntax. You can use them to include IE7-specific CSS, e.g.
<!--[if IE 7]>
<link rel="stylesheet" type="text/css" href="ie7.css" />
<![endif]-->
There’s no equivalent in CSS, unfortunately. But, as mentioned in other answers, there are some valid CSS hacks you can use to target CSS rules as just IE 7.
I personally prefer the conditional comment syntax as it’s a bit more explicit, but you can make the hacks explicit with comments.
If you don't want a separate stylesheet for IE hacks, here's another way doing it with using conditional comments:
<!--[if lt IE 7]><body class="ie6"><![endif]-->
<!--[if (gte IE 7)&(lt IE 8)]><body class="ie7"><![endif]-->
<!--[if gte IE 8]><!--><body><!--<![endif]-->
...page content...
</body>
This give IE6, IE7 and [all other browsers] a different body element class. Now you can write rules like:
body.ie7 div.scroll { padding-bottom: 16px; }
are expressions valid? if so:
cssAttr: expression( /msie 7/i.test( navigator.userAgent ) ? '#ie7val' : '#0th3r1' );
I highly doubt they are though, and technically that's CSS, but it's really JavaScript in disguise!