I was talking about is making up a new markup language, which doesn't have to inherit the distinctions and behaviors of HTML.

If, in this language, you wanted to have this feature in combination with what I proposed, you could simply mark the attribute-like children, to inform the engine that it should apply different defaults for them and unmarked children that it doesn't recognize.

This is what I do in the first variant of my language:

  a [
    href=[...]
    [...]
  ]

the `=` appended to `href` marks it as an attribute.

This certainly simplifies things when translating to HTML.

But if we were not constrained by the legacy of HTML then perhaps bothering with this when designing a new language would be unnecessary. Maybe not having this default behavior does not matter in practice and you can have a simpler language without it.

To determine whether that's the case it would help to answer: when is the feature you described useful in HTML? And also: when is this feature harmful?