From http://dev.w3.org/html5/spec-preview/syntax.html#element-restrictions:

"A single newline may be placed immediately after the start tag of pre and textarea elements. This does not affect the processing of the element. The otherwise optional newline must be included if the element's contents themselves start with a newline (because otherwise the leading newline in the contents would be treated like the optional newline, and ignored)."

This is about the syntax, so applies to serializing and parsing. A serializer must emit an additional newline if the pre content (without the optional newline) in the DOM starts with a newline. Sadly, the innerHTML method in modern browsers does not seem to implement this, so we have to work around it.

Fix merged in https://gerrit.wikimedia.org/r/#/c/36923/