After checking a few dozen articles I am still in the dark how to actually implement a responsive ux-design (I really understood the layout part). A simple example: Bootstrap (CSS framework) collapses toolbar buttons into one drop-down button on mobiles. That's all I need! I'd like to understand just the basics of this very behavior. Does it requires only CSS, or do I need JavaScript too? I actually don't know any other way than JavaScript.
In my apps (IDEs), I usually have lots of toolbar buttons and at a certain breakpoint or width I guess I will have to put them just into a dropdown button, in one way or another, right? The obvious things such as re-arranging layout containers per break-point/width are a piece of cake with CSS; at least this is how it looks.
Update, I am working only with JS based widgets. Mostly generating all HTML.