I am presently working on a website for which I had declared no DOCTYPE. Of course it was impossible to validate the HTML without a DOCTYPE, so I added one (HTML 4.01 Transitional). The HTML now validates and the CSS validates as CSS2.1. However, before adding the DOCTYPE the site worked across all the browsers I tested (IE8, Firefox, Chrome, Safari) and now it only works for IE. All other browsers completely ignore the CSS.
Rob, you were correct on all accounts. I am using variables inside my CSS and adding the header to the style sheet has corrected all the problems. Thank you, sir, for your help in my hour of confusion.