consider whether document-level language fallback rules make sense in all cases

[dbaron]

What is the issue with the HTML Standard?

As discussed in #9796 (comment) the rules for document-level language fallback in the section on lang and xml:lang attributes may not make sense for all cases. @annevk wrote:

I wonder if this fallback makes sense in all cases. In particular for synthetic documents or an XHR document or some such.

in reference to the existing text:

If there is a pragma-set default language set, then that is the language of the node. If there is no pragma-set default language set, then language information from a higher-level protocol (such as HTTP), if any, must be used as the final fallback language instead. In the absence of any such language information, and in cases where the higher-level protocol reports multiple languages, the language of the node is unknown, and the corresponding language tag is the empty string.

It's also possible that (given the refactoring in that PR into a <dl class="switch"> that there could be some useful editorial refactoring of this large-ish "Otherwise" step into more than one step. However, I'm not sure whether or not that's desirable since all the other steps in the switch are conditions about the node itself.