IMO today we are in a transition time on front-end development. HTML, JS and CSS has evolved a lot and the traditional way of building front-end separating design and implementation must evolve as well.
Particularly, I see the front-end moving to a component way of development (like React and other frameworks do). Who creates a date-time picker from scratch today? When we think about components, it makes sense to combine JS, HTML and CSS in one single stack for that component, and this way of programming goes against the traditional HTML+CSS design.
My current project uses React + Bootstrap + webpack + ES6. I'm very happy with that, but I see no other way of integrating designers but introducing them to React.
Designers don't need to know React deeply, but at least declare a new component and implement its basic render reusing other components - that won't hurt much.
But using React or not, I can't see designers not using a lot of JS. Just check the amount of JS code that comes with these Bootstrap templates.