I don't think this needs to be a UA Guidelines requirement since it affects all users. One goal of 2.1 is to make equivalent alternatives readily available in the same view. Ensure that the user has access to all content. I think the objectives are valid, but may need to go somewhere outside of G2. Provide synchronized views of content.
Most browsers already provide at least one view.
It's important that synchronization is important if you provide an outline view. Issue of different levels of detail in views and how those are coordinated. Yes, but P2. Yes, at least P2. E.g., from a link list, you need to be able to find out where the link occurs on the page.