Setting innerHTML on the <html> element parses the fragment in its own context
(added in a2fd4d1), which runs html5ever's "before head" insertion mode and
synthesizes both <head> and <body> under the fragment root. The importer's
helper walked into the root <html> and, if it found a <body> child, returned
that body -- so import_children_from imported only the body's children and any
assigned <head> content (<title>, <meta>, <link>) was silently dropped.
A <body> child of the fragment root only appears in this <html>-context case;
every other element context leaves parsed content directly under the root and
already fell through to returning it. Rename find_body_or_root -> fragment_root
and return the root <html> element so both head and body children import.
Behavior for all non-<html> contexts is unchanged.
Fixes#556
Andrey Mnatsakanov
d0d8617ca6