How?

By making text flow to 100% of the width you can easily resize the browser window for everyone's perfect reading width.

I usually run my browser tiled to 50% of the screen, the new design wastes a lot of space at that size (you have to shrink to really narrow width before it enters 'mobile mode' and uses 100% of the width again - I mean it's pretty insane, you can see more text at say 30% width, but at 40%-60% it pops out that useless white space padding on the left leading to a narrower effective text width)