Assume that these are the style rules that define a page's layout elements:

header $\{$ text$-$align:center; $\}$

nav $\{\}$

#left $\{$ float:left; width: $20\%\}$

#main $\{$ float:right; width: $80\%\}$

footer float: clear both; $\}$

The # in front of the 'left' and 'main' style rulesets indicates that those are style rules.

Element$-$based

Class$-$based

DIV$-$based

ID$-$based