> For flexibility, attributes names containing underscores (the U+005F LOW LINE character) are also reserved for experimental purposes and are guaranteed to never be formally added to the HTML language.

> Note: Pages that use such attributes are by definition non-conforming.

in a broader context of "experimental purposes" being for "vendor-specific proprietary user agent extensions to this specification", which "are strongly discouraged. Documents must not use such extensions, as doing so reduces interoperability and fragments the user base, allowing only users of specific user agents to access the content in question."

Is that it? That seems different, but I don't see anything else similar. Or am I looking in the wrong spec?

There's also https://html.spec.whatwg.org/#attribute-name-state which defines attribute names as not inclusive of underscores.