If you code by hand, you might sometimes decide to leave out the quotes around the parameter values such as 80% in the above example. Although most browsers will excuse this, it is incorrect not to place quotes around values that aren't plain numeric values.
<HR WIDTH = 500>
is correct, however the following is incorrect:
<HR WIDTH = 80%>
because, the latter contains a non-numeric character -- "%" Following is the proper way to specify your HTML parameters:
<HR WIDTH = "80%">