I have a vague memory of CSS Regions being talked about, but had completely forgotten them. It turns out that's likely because the spec has effectively been pulled. I think it's technically still out there, but IE11 is the only browser that still has any compatibility with them, so you can effectively treat it as DOA. That's a little bit of a shame, as I quite like the idea of being able to flow content between sections of a page, but Bruce does a good job of highlighting why it didn't work out and what a future second attempt might do better.

On the positive use-cases for Regions:

Some mobile navigations that we see on the web are a duplicated version of the desktop navigation, Regions will give us the ability to flow them instead of duplicated the general structure

The basic idea was to move things from the sidebar into specific regions throughout the content and then making the asides and ads more content aware for smaller screens.

On their downfall (one of the reasons that Safari and Google both pushed to remove them from browsers as the mobile web took off):

Magazine layouts on the web, while they look cool - would inhibit a user from finding what they were looking for in the first place.