Comment by jeroenhd

My guess is that they're fine with WebBluetooth/USB/FileSystem/etc. because the code for the new standard is recent and sticks with modern security sensibilities.

XSLT (and basically anything else that existed when HTML5 turned ten years old) is old code using old quality standards and old APIs that still need to be maintained. Browsers can rewrite them to be all new and modern, but it's a job very few people are interested in (and Google's internal structure heavily prioritizes developing new things over maintaining old stuff).

Nobody is making a promotion by modernizing the XSLT parser. Very few people even use XSLT in their day to day, and the biggest product of the spec is a competitor to at least three of the four major browser manufacturers.

XSLT is an example of exciting tech that failed. WebSerial is exciting tech that can still prove itself somehow.

The corporate installations still doing XSLT will get stuck running an LTS browser like they did with IE11 and the many now failed strains of technology that still supports (anyone remember ActiveX?).