Hello Remi, very interesting topic!
To give you some data, about 10–15% of our students have design and/or front-end background, having worked for web-agency or as freelance before joining Le Wagon.
I personally think you need to be a “full-stack designer” nowadays, especially when you work for a startup. And for that you need to know front-end dev at least.
Also, applying code principles to design & front-end (like DRY, modularity & reusability) makes you much more efficient. Taking the example of CSS, thinking your code by “components” (I have a Youtube workshop on this topic, in French), naming your CSS class with good conventions so that they are easy to re-use, etc. All these “dev practices” make a big difference.
I plan to write a Medium post about a “developer approach of design”. I’ll keep you posted on that!