* CSS: learn how to use flexbox and grids, the rest will follow.
* HTML5: this is pretty straight forward to learn IMO, maybe someone has a good resource? (that does not talk about JS at all preferably)
* JS: This one is tricky, if you really want to learn more about JS it is its own big world. Two advice I would give: learn the latest JS version and use a transpiler. The most popular framework seems to be React, so learn how to use that.
That's the one part of the stack I still need to learn (becuase jQuery is not trendy anymore apparently)
It works quite nice, only some old browsers that don't support it properly. Still advised to check for grid support via css, but don't think this will be required for long.