None of this seems particularly different from doing <i>spans</i> inline (what if you forget to close one? or they overlap?), but it has a major benefit of being very backwards compatible, supporting future variants in parallel for gradual migration, and doesn't require any changes to search tools. And separating markup instructions from content is generally a very good idea.

And you could use markdown. Just get the rendered spanned text result and transpile it. This would be true for any editor as well, just get convert it to spanned text, edit, convert back.

And with HTML you'll still have to specify what subset you support, and many tools won't support that either. Markdown suffers from this too, by supporting embedded HTML, though at least commonmark is pretty baseline and well supported.