Q: How do you make an image maintain an aspect ratio and fill up the width or height of the screen, whichever comes first? A: Nested div hell and JavaScript.

Q: How do you make a scaled background image stay put even when the keyboard input pops up on a phone? A: Supreme JavaScript, CSS, and div hell.

Q: How do you center a paragraph of text vertically in a div? A: Nested div hell.

I'd say FlexBox is pretty inadequate. Why can't we have things like:

    #my-video {width:80%;height:calc(width*2/3);}