It's easy overlooking the basics. One must be able to clearly explain how the web works, using the dedicated vocabulary.
I recommend carefully reading the following resources:
The HTTP protocol
A browser mostly "speaks" HTTP. It will craft HTTP requests for you when accessing or interacting with a website. When using JavaScript, you might also create custom HTTP requests. HTTP is the underlying language of the web.
- https://www3.ntu.edu.sg/home/ehchua/programming/webprogramming/HTTP_Basics.html
- http://www.tutorialspoint.com/http/
Practicing with curl is a good idea:
- https://gist.github.com/caspyin/2288960 (replace "caspyin" with your GitHub username, and add the
-voption to commands to show further details about the HTTP requests)
The REST architecture
A very commonly used architecture for client/server communication.
Introduction to Web development on MDN
About HTML, the sections:
- The basics of Hypertext Mark-up Langage (HTML)
- Basic structure of a web page
- Fundamental HTML elements
About CSS, the sections:
- Getting started with CSS
- Common CSS questions
- Intermediate CSS concepts
About JavaScript, the sections:
- Getting started with JavaScript
- Eloquent JavaScript (a free book)
- All the Intermediate sections