For example, CSS grids are a new browser feature we use for the entire layout of the editor. Grids are designed specifically for this usage in mind, and if we wanted to make it an optional feature we'd end up with div soup.
Grids are weeks old for public use - it's odd to think that this beta literally couldn't have happened even a month earlier, as the support for C3 wouldn't have existed without dev flags or Canary. Cutting edge indeed.
At my job we still have to go through a SWOT to see if we can use flexbox...