Update spec language around 1-UA mode to reflect Multi-Screen Window Placement

[markafoltz]

In the future we should discuss how to update the spec language to account for the fact that since the spec was written, there are new ways for documents to project locally rendered content to secondary displays, including Multi-Screen Window Placement and the Site-Initiated Mirroring API.

These new capabilities could push 1-UA mode to a secondary role in the implementation, including possibly dropping it from the spec entirely.

My interpretation of the current spec is that 1-UA mode is not part of the normative language of the specification, and is more of a guidance for implementers.

Note that Chrome does currently implement 1-UA mode, so we don't have any immediate proposals for changes here. However we will update this issue if our plans change.

[tidoust]

These new capabilities could push 1-UA mode to a secondary role in the implementation, including possibly dropping it from the spec entirely.

That's fine. I just note that the spec mentions the need to have at least one implementation of the 1-UA mode in its CR exit criteria