HR html tag not printing when using CSS

I was using the <hr/> tag in an html page, and it was rendering fine when viewed in the browser.  However, when I printed the page, nothing printed.   The HR tag (the line itself) did not print.   I was using CSS to style the HR tag.
 
It turns out the CSS syntax seems to be very picky.    After playing around, I finally got this exact CSS code to work:
 
hr {
  border-color: #000000;
  border-style: solid;
  border-width: 2px 0 0;
  clear: both;
  margin: 0 0 0px;
  height: 0;
}
 
The line both viewed properly in the browser and it printed properly as well.
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