Heck, I would have been happy to have had flexbox day one.

I think flexbox covers 'most' folks layout needs fairly easily, and you can learn it fairly easily and get a lot out of even basic flexbox knowledge.

Tables for layout were horrific. We talk about div spam these days but at least they're all just divs and divs follow divs rules. Unlike divs table, tr, td are all different and don't follow the same rules. And the result of table layouts was table inside tr,td in another table and in another table is its own nightmare and gets ultra inflexible and could require whole hog layout changes just to move a widget if the widget was just a bit too big. Tables were horrible.

Just flexbox day one would have been amazing.