Not sure how a subset would work in practice. Though I can imagine a javascript gemini client (as perverse as it sounds...) that could be used as a tunnel into the gemini world in cases where you just want to look something up quickly and/or don't have or want to install a proper client
What I'm imagining is a specification, defining which subset of HTML should be included, together with a <meta> tag which indicates a document's intention to be compliant with the standard.
It sounds like you're expressing disagreement with Gemini's chosen feature-set, but the question being discussed here is whether it would make more sense to use a subset of HTML rather than an incompatible format.
The point of using an HTML subset would be to retain compatibility with conventional heavyweight web-browsers. A feature-rich YAML-based scheme would be neither minimalistic, nor natively web-compatible.
If most users would end up browsing gemini with a regular browser that sort of thing would be commonplace.