Class vs. ID
Above, we mentioned that there were two constructs for custom selectors -- "class" and "id." You've seen that a class can be applied as many times as you like on a page. IDs work almost the same way, but apply to just one element on a page. When creating rules for IDs, simply prepend the selector name with "#" rather than "." For example:
Since you will probably only ever have one footer on a page, it makes sense to use this as an ID rather than as a class. You would invoke this ruleset in your document like this:
Recap: When writing CSS rules, you don't have to specify an existing HTML tag as a selector. If you start any word in a ruleset with a dot ("."), it becomes a custom selector called a "class", which can then be applied to anything on the page. In our example, we used the word ".alert" in our ruleset, prepended it with a dot, and it became the rule for the selector "alert". Similarly, we can address a single element on a page by referencing with an ID rather than a class. Rules for IDs use the "#" syntax rather than the "." syntax. In general, you'll use IDs when elements are guaranteed to be unique on a page, and classes when they're not.
The ability to style any existing HTML element, combined with the ability to create custom, named selectors, gives you complete design and display flexibility.