Using

#my-div {
height: 7px;
background-color: #29425A;
_background-color: #29425A;
}

Shows correctly in Firefox (just a line across the page), but is blank in IE. Also tried it with * and #.

IE 6 & 7 are still used by a significant portion of the browser market share, and although I pity all of us web developers who have to deal with that, it’s not something we can simply snub our noses at. I personally dislike IE conditionals, as do most of the designers I’ve worked with over the years. Maintaining 2-3 extra style sheets beyond the first one is far more trouble than it’s worth. If your main style sheet is such a mess that you can’t keep track of what’s going on, then I suggest you take some time to clean it up rather than make more of them.

I work for a company whose websites are viewed by countries who have very poor internet connections, and who primarily use IE. Should I tell them to “Suck it, you IE-using-cornholes!” or should I keep my http requests down to a minimum, with the only real downside being a style sheet that is marginally larger with simple and easy to manage hacks?

For me, it’s no contest. Haxxors all the way.

And as for validating CSS, what’s the point? HTML is a structured and fully supported markup language that serves as the foundation for all web content. It HAS to be valid to ensure that all the pieces stacked on top of it will work. CSS… it’s a paint job. And although I really do like paint and for things to look pretty, I’m not about to validate that Home Depot gave me the stirring stick without the asterisk on it.