XML is notoriously expensive to properly parse in many languages. Basically, the entire world centers around 3 open source implementations (libxml2, expat and Xerces), if you want to get anywhere close to actual compliance. Even with them, you might hit challenges (libxml2 was largely unmaintained recently, yet it is the basis for many bindings in other languages).
The main property of SGML-derived languages is that they make "list" a first class object, and nesting second class (by requiring "end" tags), and have two axes for adding metadata: one being the tag name, another being attributes.
So while it is a suitable DSL for many things (it is also seeing new life in web components definition), we are mostly only talking about XML-lookalike language, and not XML proper. If you go XML proper, you need to throw "cheap" out the window.
Another comment to make here is that you can have an imperative looking DSL that is interpreted as a declarative one: nothing really stops you from saying that
totalOwed = totalTax - totalPayments
totalTax = tentativeTaxNetNonRefundableCredits + totalOtherTaxes
totalPayments = totalEstimatedTaxesPaid +
totalTaxesPaidOnSocialSecurityIncome +
totalRefundableCredits
means exactly the same as the XML-alike DSL you've got.
One declarative language looking like an imperative language but really using "equations" which I know about is METAFONT. See eg. https://en.wikipedia.org/wiki/Metafont#Example (the example might not demonstrate it well, but you can reorder all equations and it should produce exactly the same result).
The main property of SGML-derived languages is that they make "list" a first class object, and nesting second class (by requiring "end" tags), and have two axes for adding metadata: one being the tag name, another being attributes.
So while it is a suitable DSL for many things (it is also seeing new life in web components definition), we are mostly only talking about XML-lookalike language, and not XML proper. If you go XML proper, you need to throw "cheap" out the window.
Another comment to make here is that you can have an imperative looking DSL that is interpreted as a declarative one: nothing really stops you from saying that
means exactly the same as the XML-alike DSL you've got.One declarative language looking like an imperative language but really using "equations" which I know about is METAFONT. See eg. https://en.wikipedia.org/wiki/Metafont#Example (the example might not demonstrate it well, but you can reorder all equations and it should produce exactly the same result).