There are two changes here:
1) We allow setting .body even if the root element is not an <html:html>. This is what the spec says to do, and what we used to do before the changes in bug
366200. No tests for this yet, pending https://github.com/whatwg/html/issues/3403 getting resolved.
2) We use GetBody(), not GetBodyElement(), to look for an existing thing to replace. This matters if there are <frameset>s involved.
Tag UXP Issue #1344
Tag UXP Issue #252
