Q: Validation errors due to *width IE-hack I'm using this tool to validate a CSS file:  http://jigsaw.w3.org/css-validator/
I have 23 errors in my CSS file and more than half look like this:
Line 46  #left_navigation    Parse Error ;*width: 175px;

The actual CSS looks like this:
#left_navigation {
    float: left;
    width: 165px;
    *width: 175px;
    margin:0px;
    padding:12px 5px 5px 5px;
}

The error is due to the asterisk setting the width. I didn't put this here originally, but it's clear that its needed to deal with some display problems in IE.
So my question is: do I care about these errors?  Is there another/better way to be doing this?  I can't just take it off because bad things happen in IE when we do this.

A: 
Do I care about these errors?

I'm not sure. Do you?
There's an alternative though. Make sure your pages have something along these lines:
<!--[if lt IE 7 ]> <html class="ie ie6" lang="en"> <![endif]-->
<!--[if IE 7 ]>    <html class="ie ie7" lang="en"> <![endif]-->
<!--[if IE 8 ]>    <html class="ie ie8" lang="en"> <![endif]-->
<!--[if IE 9 ]>    <html class="ie ie9" lang="en"> <![endif]-->
<!--[if gt IE 9]><!--><html><!--<![endif]-->

Then you can use IEx-specific styles with custom selectors, like such:
.ie7 #left_navigation { width: 175px; }

No need for the *prop: value; hack anymore.
Alternative to the approach above, you could also use the IE-conditionals to include a seperate/additional or alternative IE stylesheet.

A: Any browser will anyway work fine. If it is not for some reason crucial to have validated CSS just ignore it.
Hard solution is to make it work properly on all browsers.Depending on required backward compatibility it might be hard.
There is also something like (Modernizr)[http://modernizr.com/] which could help you with the task with somehting like:
#left_navigation.ie{
/* IE specific code */
}

Althrough Jeoren posted ready solution:)

A: If you wanna go for a valid CSS hack, 
the following allow you to include a solution without necessity to include the whole modernizr script or changing the HTML (pre-)header by adding IE conditional comments:
* html #left_navigation { width: 175px; } for IE4-6 and 
*+html #left_navigation { width: 175px; } for IE7
See http://en.wikipedia.org/wiki/CSS_filter#Star_hack
