Implement <br>

[SimonSapin]

Based on what the HTML spec says as of this writing, I previously said that the <br> element could be implemented as br { content: '\A'; white-space: pre; } the user-agent stylesheet and not require any specific Rust code.

It turns out that what other browsers do is slightly more complicated, so this is still an open issue at the spec level. Discussion starts here:

http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2014-January/211267.html

Although maybe we don’t need to get all the details of this behavior right, for a first approximation.

[SimonSapin]

#1638 adds a hack to the user-agent stylesheet to kind of simulate <br> without the content property. (It looks like it could break in all kinds of interesting ways in corner cases.) It should be removed once we have something better.

[mbrubeck]

#3163 added a pure CSS implementation that's similar to the one in the HTML spec. However, depending on how the spec issue is resolved, we may still need to change this to match other browsers' behavior.