[css-display] Explaining <br>

At the January f2f, we figured out a decent hack that seems to explain
the rendering of <br> properly:

br {
  display-box: contents;
  content: "\a";
  white-space: pre;
}

However, when Hixie was planning to add this to the HTML style sheet,
dbaron objected to it on performance reasons:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=25503

So, David, how bad is this?  Should we go with a different solution,
like a dedicated 'display' value just for <br> (Hixie suggested
"display: newline;" in the linked bug).

~TJ