The output of web design is, ultimately, HTML and CSS, so what reason could a professional have to work with tools that can only yield an approximation of what they are paid to do? In my view, a designer without full mastery of at least HTML and CSS doesn't really deserve his title.
Going a little farther: with some knowledge of JavaScript, JSON and how to transform it (map, reduce, filter, sort), a designer can ask for some sample data and come up with a functioning prototype that offers truly important insights into the problems his design is supposed to solve. Client-side frameworks like Svelte.js are making all this very easy.
Nothing is more maddening, and a waste of time and money, than a Photoshop mockup using rudimentary, and generally too self-complacent, content.