Selecting Elements by ID or Class
If you've labeled elements with an id or class (see page 63), you can use that criteria in a selector to apply formatting to only those elements that are so labeled.
Figure 9.9. The division with an id of gaudi encloses almost the entire page (everything but the initial h1).
To select elements to format based on their id:
Figure 9.10. This selector will choose the div element with an id equal to "gaudi".
To select elements to format based on their class: