Specifically ID and CLASS attributes are case sensitive when it comes to CSS.  If you take a look at the HTML 4 spec you'll notice that both ID and CSS have [CS] annotation for case-sensitive. 

Although this has always been the case, very few developers know this as a fact.  Its obvious Microsoft missed this in the first few HTML 4 compatible versions of IE.  In fact it wasn't until IE 6, when run in strict mode, would IE in fact honor the case sensitive nature of the CLASS and ID attribute.

I think its because of the early IE 'quirks' mode implementation that very few developers actually know that ID and CLASS are in fact case sensitive. They may guess correctly, but very few actually know this from experience.  A simple quiz at work today confirmed my suspicion.

Well after today, I know from experience that CSS is case sensitive.  <div class="msg-Error"/>  is not the same as  <div class="msg-error"/>.

So in summary if your CSS is not getting applied as expected you might want to check the casing between your HTML and your CSS.