Viewport units (vh/vw) were a mistake to add to CSS; slower than alternatives (layout systems) assuming good implementation

@davidbaron Interesting. What makes them slow? What is a "layout system" in this context? Might need > 140 chars.

@SimonSapin Viewport units need recomputation of style data for size changes plus the work the layout system (block, flex, &c) requires.

@davidbaron @SimonSapin size changes in practice are mainly device orientation switches. Devs frantically resizing the window doesn't count