Processing instructions (syntax: <?target data>) are an existing DOM construct, exposed in XML, that allows node objects that are not elements but can have some semantic meaning for the processing of a document. Processing instructions are parsed by the HTML parser, and receive an attribute API similar to elements to mutate their data.
The work on out of order streaming showed that defining declarative ranges with elements is insufficient, and there's need to be a "lighter" declarative way to denote ranges, that doesn't e.g. affect CSS relationships. This seems to unlock some future use-cases, such as declaratively denoting CSS custom highlights, and controlling the streaming/buffering of an HTML response.
Explainers: https://github.com/WICG/declarative-partial-updates/blob/main/patching-explainer.md#proposed-markup