Do the same thing you might do when browsing open source code on github. Go to websites of well known companies or by people that provide web design services, view source, look at a feature that doesn't make sense at first, and then search on Google or Mozilla Developer Network or Stackoverflow for more details, and then write a practice page in a basic text editor.
As far as design, you cannot go wrong with bootstrap. However, to really learn design, do not study web design. Study things like Bahaus, billboard design, marketing and messaging and PR, how to compose for simplicity, SEO optimization techniques and their influence on design, etc. Seriously, the people that learn how to make websites and want to tell you how to make websites absolutely and hilariously SUCK at design.