What is the best source to learn website development?

Depends. level of skill you already have, languages you prefer over languages people shove down your throat. Freecodecamp is very js orientated. codeacademy will laydown some general knowledge. tuts+ network old stuff like 4 years ago is a good start. coursera and similar free course sites. youtube obviously. find something that seems to cover a big site build do that else will end up poking at bits and pieces never sure how these fancy new things fit for years to come.

I agree with the two choices given above. There's also:

Tutorialspoint (my favorite source)
W3schools
Sololearn
Khan academy
And Coursera

I feel old now, go buy a book in the languages you want to get into.

Web is a dauntingly large and broad topic now. What do you want to focus on?

Idk, but i feel like youtube is better sometimes.

But if you want to start from 0, the recommended websites in the previous comments are good.

@OneOrZero +1
and later csstricks.
Please lool at their flexbox guide later on ^^

W3schools covers most of web development. YouTube videos really helps, like thenewboston channel