Here are some great (FREE) resources for learning HTML & CSS
Codecademy - HTML & CSS course
Shay Howe - Learn to Code HTML & CSS
FreeCodeCamp Responsive Web Design Certification (300 hours)
Udacity - Intro to HTML and CSS
Khan Academy - Intro to HTML/CSS
MDN - Flexbox
Flexbox - Flexbox Froggy
Flexbox - Flexbox Zombies
MDN - CSS Selectors
CSS Diner
MDN - CSS Grid
CSS Grid Garden