XSLT is mature, widely accepted standard.

It can be used in browsers (even in old IE) and on server side (nginx has XSLT module, it can be used from programming laguages, of course). Its implementations are compiled and, therefore, should be much faster than Python or JS. JS implementation Saxon JS can be used, at least, as fallback. Jinja, Angular, Ruby's Slim, ASP and PHP templating are not even close.

XSL template can be easily validated in IDE. How many IDEs can help with Jinja or Angular?

Looks like it's perfect idea to decompose UI and data with XSLT.

Admittedly, implementations can give different results in some corner cases, but it's a problem only with templating on client side. And it's same with HTML, CSS and everything else that is done on client side.

So, why not XSLT?

XSLT is a mature, widely accepted standard.

It can be used in browsers (even in old IE) and on the server side (nginx has an XSLT module, which can be used from programming languages, of course). Its implementations are compiled and, therefore, should be much faster than Python or JS. The JS implementation Saxon JS can be used, at least, as a fallback. Jinja, Angular, Ruby's Slim, ASP and PHP templating are not even close.

An XSL template can be easily validated in an IDE. How many IDEs can help with Jinja or Angular?

It looks like it's a perfect idea to decompose UI and data with XSLT.

Admittedly, implementations can give different results in some corner cases, but it's a problem only with templating on the client side. And it's same with HTML, CSS and everything else that is done on the client side.

So, why not XSLT?