From a blog:
The basic idea with CSS expressions is that you will have calculation and dynamic values for properties in the CSS code, something that people have found very useful. A simple example can be implementing max-width behavior in IE 6:
width: expression(document.body.clientWidth > 1100)? "1100px" : "auto";
This is the first time I read about them. It seems IE used to support CSS expressions but dropped them in IE8. What other browsers still use them and are they generally a good or bad thing?
The blog post I got this from says the alternative is Javascript, but I thought CSS was more supported and therefore better than Javascript.
AFAIK, it was only ever IE6/7 (maybe) 5.
I never thought they were a good thing. May as well just use JavaScript directly.
They are in fact implemented in JavaScript, and I'm pretty sure disabling JS disables these expressions.
The sample you posted...
width: expression(document.body.clientWidth > 1100)? "1100px" : "auto";
...is just a ternary operator that says If the width is larger than 1100px, set it 1100px, otherwise set property to auto.
To finish, no scripting language on the web is more widely supported than JavaScript.