This contains an (intentional) behavior change, which is that we always copy
(i.e. don't reparse) style attributes, even across documents.
XUL and HTML already had this behavior. This makes stuff like SVG and MathML
consistent with that.
Depends on D44128
Differential Revision: https://phabricator.services.mozilla.com/D44129
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extra : moz-landing-system : lando