I learnt the basis of HTML5 & CSS2-CSS3 in a course in 2013/2014, but you can do it from scratch with a lot of online resources. I found a very good article From zero to Front-end hero where it explains how to start and keep learning HTML & CSS:

• Getting started:
  • Introduction to HTML
  • Introduction to CSS
• Beginning courses:
  • Introduction to HTML and CSS
  • JavaScript, HTML and CSS Web Development
  • The Elements of Web Design
  • Learn HTML/CSS (spanish)
  • HTML5 semantics
  • CSS3 tutorials
• Advanced course:
  • Learn advanced HTML & CSS