However, the compatibility differences among various browsers are significant, and the adaptation difficulty is also very high.
Typically browser engines that don't support an XML specification version will do the proscribed thing and ignore it. So some old Series 60 WAP browser (were you to find such a thing today) would ignore the XSL stylesheet and display the raw XML or hand it to the registered handler for the content type.
The world went bonkers supporting XML on every computing platform under the Sun for nearly a decade.