Beginner
Learn HTML (www.codecademy.com)
HTML Content (www.codecademy.com)
Front End Interview Handbook (github.com)
Organizing HTML & CSS (www.codecademy.com)
HTML Fundamentals (www.codecademy.com)
HTML Best Practices (github.com)
Learn HTML in 12 Minutes (www.youtube.com)
w3schools (www.w3schools.com)
Marksheet (marksheet.io)
HTML Beginner Tutorial (htmldog.com)
How to Use the HTML5 Sectioning Elements (blog.teamtreehouse.com)
Intermediate
HTML Intermediate Tutorial (htmldog.com)
Advanced
HTML Advanced Tutorial (htmldog.com)