w3schools.com
URL
w3schools.comDescription
It contains a vast amount of information on every web development topic and is good to begin with.
Specific sections on:
HTML (https://www.w3schools.com/html),
CSS (https://www.w3schools.com/css),
Javascript (https://www.w3schools.com/js)
PHP (https://www.w3schools.com/php)
