https://www.w3schools.com/js/default.asp (W3Schools)
https://developer.mozilla.org/uk/ (MDN)
https://github.com/getify/You-Dont-Know-JS (YDKJS)
Quokka.js plugin for Visual Studio Code
HTML https://www.w3schools.com/quiztest/quiztest.asp?Qtest=HTML
CSS https://www.w3schools.com/quiztest/quiztest.asp?qtest=CSS
JavaScript https://www.w3schools.com/quiztest/quiztest.asp?qtest=JavaScript