Comment by dfabulich

XSL is a functional programming language, not a declarative language. When you xsl:apply-template, you're calling a function.

Functional programming languages can often feel declarative. When XSL is doing trivial, functional transformations, when you keep your hands off of xsl:for-each, XSL feels declarative, and doesn't feel that bad.

The problem is: no clean API is perfectly shaped for UI, so you always wind up having to do arbitrary, non-trivial transformations with tricky uses of for-each to make the output HTML satisfy user requirements.

XSL's "escape hatch" is to allow arbitrary Turing-complete transformations. This was always intended to exist, to make easy transformations easy and hard transformations possible.

You basically never need to write Turing-complete code in a type system, but in any meaningful XSL project you will absolutely need to write Turing-complete XSL.

XSL's escape hatch is always needed, but it's absolutely terrible, especially compared to JS, especially compared to modern frameworks. This is why JS remained popular, but XSL dwindled.