Css breakpoints.
Check out responsiveness course from Kevin Powell. Short, but does a good job of making someone understand the responsiveness and writing good html in general.
I have seen people write so bad html templates and css in production code. Please don't turn out to be like those and give proper importance to html and css.
Yes. And don't make the mistake of running after frameworks like what many are suggesting.
Just get a good enough understanding of html, css and Javascript. Frameworks can wait and can easily be learned and un-learned. Don't run after any of the fancy stuffs like bootstrap, React, Angular etc until you are comfortable enough with html, css and js.