Both Firefox and Safari are vulnerable to XSS if we use an inert document
created via `document.implementation.createHTMLDocument()`.
Now we check for those vulnerabilities and then use a DOMParser or XHR
strategy if needed.
Further the platform-server has its own library for parsing HTML, so we
sniff for that (by checking whether DOMParser exists) and fall back to
the standard strategy.
Thanks to @cure53 for the heads up on this issue.
