How to learn web design if backround is on mathematics and data analysis? What would one suggest to learn web development? I was educated to mathematician and have done data-analysis. Now I was asked to do web pages, and I would like to practice that. However, it seems to be tricky to learn how to make sites to look good in both laptops and mobile browsers. Therefore, I would like to know good books, lecture notes, studying roadmap etc. to learn responsive web development by HTML and CSS. I like to learn those technologies first as the free host github sites supports those.
Just about everything you need can be found in one or another book. O'Reilly has a large selection of reputable work. (No affiliation) But a topical search online or at Amazon will get you what you need. 

If you prefer video, a search at YouTube, say, will turn up lots of possibilities. 

Prefer recent books and videos, however. Straight html is seldom used for serious work these days.