HTTPLeaks
Many different ways to leak data from html
https://github.com/cure53/HTTPLeaks
Page with general CTF/PenTesting guides, tips and tricks
Awesome list with pentest links, tips, cheatsheets
https://github.com/coreb1t/awesome-pentest-cheat-sheets
PortSwigger Web Security Labs - A MUST for web penetration testing!
All labs | Web Security Academy
Stanford - CS 253 Web Security