As far as I can tell, this is not actually allowed by the current proposal. The definition of the "find markers" algorithm in https://github.com/whatwg/html/pull/11818 requires that the <?end> marker be a sibling of the <?start> marker in the DOM, so they can't have different parents like this.

Well, you're allowed to write it, but the browser will just ignore the <?end> marker. The range will end at the </em> tag, the same place https://github.com/WICG/declarative-partial-updates/blob/mai... says it goes by default if you don't write the <?end> at all. (That default wouldn't make a ton of sense if patch ranges were allowed to break out of their parent elements like this.)

Jake Archibald thinks it'd be cool to use a similar syntax to do CSS highlights, but there are other problems with that (like not being able to create overlapping ranges), and he's not on the Chrome team that's pushing this proposal.