Once of the primary advantages of CSS particularly to designers is its ability to easily apply a set of styles to all elements of the same type. Unimpressed? Consider this: by editing a single line of CSS, you can change the colors of all your headings. Don't like the blue you're using? Change that one line of code, and they can all be purple, yellow, maroon, or any other color you desire. That lets you, the designer, focus on design, rather than grunt work. The next time you're in a meeting and someone wants to see headings with a different shade of green, just edit your style and hit Reload. Voilà! The results are accomplished in seconds and are there for everyone to see.
Of course, CSS can't solve all your problems you can't use it to change the color of your GIFs, for example but it can make some global changes much easier. So let's begin with selectors and structure.