I see so many websites trying to implement their own UI behaviors and getting it wrong, not scrolling smoothly, showing a modal window partially off screen in certain scenarios, preventing you from zooming out on a smartphone while overflowing the text outside of the screen, menus that are either not touch or mouse friendly, etc.
Giving UI developpers only html and css is like telling a graphic designer: all you need is microsoft paint, you see you can change the color of each pixel to whatever you want, total freedom! Yeah... No.