> Shipping the feature in Firefox Nightly caused at least one popular website to break.
and links to https://bugzilla.mozilla.org/show_bug.cgi?id=1443630 which points to a single site as being broken. There's no check as to the size of the impacted user base, but there is a link in the blog post to https://www.w3.org/TR/html-design-principles/#support-existi... which says:
> Existing content often relies upon expected user agent processing and behavior to function as intended. Processing requirements should be specified to ensure that user agents implementing this specification will be able to handle most existing content. In particular, it should be possible to process existing HTML documents as HTML 5 and get results that are compatible with the existing expectations of users and authors, based on the behavior of existing browsers. It should be made possible, though not necessarily required, to do this without mode switching.
> Content relying on existing browser behavior can take many forms. It may rely on elements, attributes or APIs that are part of earlier HTML specifications, but not part of HTML 5, or on features that are entirely proprietary. It may depend on specific error handling rules. In rare cases, it may depend on a feature from earlier HTML specifications not being implemented as specified
Which is the "servant-oriented" mindset I'm talking about here.
> Removing Flash broke tons of sites
Yes, but Flash wasn't part of a standard, it was an ad-hoc thing that each browser _happened_ to support (rough consensus and working code). There were no "build on this and we'll guarantee it will continue to work" agreement between authors and implementers of the web. XSLT 1.0, as painful as it is, is part of that agreement.