The fusion of HTML TEMPLATE and XSLT mix nowadays is possible. The TEMPLATE syntax can be expressed by XSLT 4.0 and HTML is treated as XSLT XDM to be run without DOM cloning.
To make it even simpler, XSLT 4.0 allows to use mixed default namespaces and omit xsl: prefix.
The DX is solvable by devtools plugin sufficient for inspection the template and engaged data.
- DCE with such template syntax: custom-element Declarative Custom Element implementation demo
- Chrome devtools pugin
Native implementation of template would bring the multithreaded performance, true template reuse for memory reduction, declarative syntax.
DCE is not only candidate for template use.
The HTML include could be expressed as template with data propagated by page and explicitly passed as attributes.
HTML page itself can utilize the template syntax to populate the application and page data.
The Declarative Data Source is a complimentary proposal to template as would be needed along with template for fully functional Declarative Web Application.