When to Use and Avoid an ID

When to Use an ID
 
Only one element on each page can be styled by each unique ID, and therefore IDs should be
reserved for unique, single-use elements such as a header or sidebar, or the main navigation or
page footer. This makes scanning your markup easier, as all ID attributes will denote unique
content areas or special regions of the page, while also providing greater flexibility for more
complex CSS application. Later in this chapter, further uses for IDs will be discussed in the
“Contextual Selectors” selection.

When to Avoid an ID
IDs must be avoided when there is more than one requirement for the CSS rule. Do not use an
ID for anything you are likely to multiply in the future, such as multiple images, link styles, or
paragraphs where more than one will need to be styled a particular way.