programming challenge sites

project euler
a website where you solve math problems with programming
codewars
an website with many coding challenges to solve
Rosetta code
an wiki with many programming challenges to solve
edabit
an website with many coding challenges
leetcode
an website with many programming problems to solve

free courses

mit opencourseware
free courses by the MIT university
harvard open courses
free courses by the harvard university
class central
a website to search free and paid courses with

learn html and css

w3schools
an website to teach html and css
mdn
a website by moezilla to teach html and css