Features come and go. (Others mentioned Flash. Java applets also were pretty mainstream for a while. Cookie rules changed. There's now more security plumbing, eg. CORS.) From a security and maintenance aspect it makes sense to get rid of libxslt and libxml.

It's hard to say how much the cost will be on users. They will need to copy the URL and put it into some site that applies the XSLT. Likely there will be some page that does this from JS and people can simply make URLs like https://blabla.gov/apply-xslt?xml=....&style=... and put those on websites, right?

> Can’t js your way into the browser opening an xml document and automatically applying the stylesheet as far as I know.

Yes, that's definitely not ideal, but maybe the result of this deprecation will be that we get some kind of handler registration. (Though I saw the comments about similar failed initiatives.)