Definition and Usage
The <hr> tag inserts a horizontal rule.
Differences Between HTML and XHTML
In HTML the <hr> tag has no end tag.
In XHTML the <hr> tag must be properly closed.
All "presentation attributes" of the hr element were deprecated in HTML 4.01.
All "presentation attributes" of the hr element are not supported in XHTML 1.0 Strict DTD.
http://www.w3schools.com/tags/tag_hr.asp