Rules, Images, and Multimedia - HTML and XHTML: The Definitive Guide - page 203

Figure 5-1. Paragraph tags give your text extra
elbowroom
This text is directly above the rule.
<hr>
And this text is immediately below.
<p>
Whereas this text will have space before the rule.
<p>
<hr>
<p>
And this text has space after the rule.
A paragraph tag following the rule tag is necessary if you want the con-
tent beneath the rule line aligned in any style other than the default left.
5.1.1.1. The size attribute
Normally, browsers render horizontal rules 2 to 3 pixels [*] thick with a
chiseled, 3D appearance, making the rule look incised into the page. You