You should be able to use qnames (foo:bar) for attributes, I think? The problem with data-* is that it's misuse, template directives aren't really data. E.g. something might actually use data-if, which would probably collide with a template directive.
There's no way writing illegal HTML is preferable to writing legal HTML.
Here's what the HTML spec[1] says about data-* attributes:
Custom data attributes are intended to store custom data, state, annotations, and similar, private to the page or application, for which there are no more appropriate attributes or elements.
1: https://html.spec.whatwg.org/multipage/dom.html#embedding-cu...
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