Often we think of using some class or id within a CSS but later on decide not to use them finally. But several times we forget to delete those classes from the CSS file. This may not be a problem for sites where the use of CSS is very less. But this may be accountable for much junk lines within your CSS which are unnecessary and unwanted.
Many times it happens that we use more than one CSS files for a site or application and copy many unused classes or ids. This way the amount of code loaded every time becomes a much considerable value. These unwanted CSS selectors should be deleted from the CSS files. To delete those unused CSS selectors you first need to identify them.