Comment by 8NNTt8z3QvLT8tp

Thing is the new APIs added to the web are generally quite well specified with quite good tests. These older legacy APIs sometimes don't even have a standard and almost definitely don't have interoperability (XSLT is an example of one that definitely does not work consistently between WebKit/Blink and Firefox).

For a new browser engine adding some of the new APIs is pretty trivial compared to debugging all the nonsense that comes from these kinds of underspecified legacy APIs. Removing XSLT from the spec and existing browsers means new ones don't feel the need to implement it. They don't need to decide which implementation to go with (use libxslt like chromium and webkit and you might match their behaviour but you also get all the same security vulns).

Frankly a modern engine could probably get by without handling XML entirely (aka no XHTML document support) and get by just fine but that's a separate discussion.