Ran across this CSS tip today, when I was researching an issue. I wanted to override a width in an element but the element already had a lot of styles applied to it and I didn’t want to create an entirely new ID.
Creating a new ID was my first instinct just because I don’t work with CSS nearly as much as I did when I first discovered it. I’m comfortable with the layout of most of my designs and just haven’t felt the need to more than tweak in years.
Really useful info, so check it out if you like to play with your CSS sometimes but want to keep it as streamlined as possible. http://css-tricks.com/multiple-class-id-selectors/