Is XML XSLT not supported?
I tried some XSLT but it looks like it's just not working. Same page renders fine in FF and comes out blank in Vivaldi.
Some testlinks where it does not work?
Make sure you’re serving every component with the right MIME type. Otherwise it will not work. I believe “text/xml” is the most widely supported. Though I agree with you, it appears to not work in Vivaldi.