The WHATWG recently proposed to “remove XSLT from the web platform”. This is not the first attempt. There’s a long discussion thread on the GitHub issue for the current proposal, including a lot of pushback. The same is true of the related issue to “remove mentions of XSLT from the html[sic] spec”.
Whether the current attempt to deprecate XSLT in the browser succeeds, or whether feedback from users will be sufficient to discourage them again, I don’t know. Regardless of the outcome, it's clear that we are heading closer and closer to a monoculture in web browsers.
Deprecating XSLT will break a lot of web pages. Thousands of Saxonica's pages, and perhaps tens of thousands of others. It will break web pages that have been quietly and usefully referenced by an unknown number of users for a couple of decades. It will break web pages that are immutable, that are not, or can no longer be, maintained because the projects have ended or the authors have died. Things will be forever lost because tens of thousands of pages aren’t enough.
I’m sure there are technical arguments in favor of removing XSLT from the browser, including arguments about code complexity or security. And I’m sure there are economic arguments about the importance of investing resources where they matter most, or at least in places offering the best return on investment. None of these arguments show consideration for the health of the web as a reliable, open set of technologies which can be easily used by individuals and large corporations alike.
Of course, building web applications with XSLT 1.0 is a considerable challenge. This is probably the main reason why most publishers delivering complex XML documents have continued to use XSLT on the server, where the technology is constantly refreshed, rather than sticking with a stagnant 25-year-old client-side implementation. Many XSLT users would happily switch to using client-side transformation if it were actively supported by the browser vendors, including support for later versions of the standard, which offer much higher productivity.
Saxonica publishes world-class implementations of XSLT for Java, C#, C, C++, Python, and JavaScript. Our JavaScript implementation runs both in the browser and on NodeJS. We support the current, modern standards and we’re actively involved in developing those standards for the future. XSLT is the most cost-effective solution for a wide range of practical use cases, and the size and breadth of Saxonica's user base reflects this fact.
XSLT will continue to survive, and thrive, even if the WHATWG insists on depriving web developers and users of the many advantages modern XSLT would offer them.