Web page rendering process

Taken almost verbatim from Friendly Bit.

1. You type a URL into your browser’s address bar.
2. The browser parses the URL to find the protocol, host, port, and path (The different parts of a URL).
3. It forms a HTTP request, assuming the URL begins with http (HTTP request format).
4. To reach the host, it first needs to translate the human readable host into an IP number, and it does this by doing a DNS lookup on the host.
5. Then a socket needs to be opened from the user’s computer to that IP number, on the port specified (most often port 80) (What is a TCP socket?).
6. When a connection is open, the HTTP request is sent to the host.
7. The host forwards the request to the server software (e.g., Nginx or Apache) configured to listen on the specified port.
8. The server inspects the request (most often only the path), and launches a server plugin needed to handle the request (plugin will depend on programming language used on the server; examples include php-fpm for PHP or WSGI for Python).
9. The plugin gets access to the full request, and starts to prepare a HTTP response.
10. To construct the response a database may be accessed. A database search is made, based on parameters in the path (or data) of the request
11. Data from the database, together with other information the plugin decides to add, is combined into a long string of text, typically HTML.
12. The plugin combines that data with some meta data (in the form of HTTP headers), and sends the HTTP response back to the browser (HTTP response format).
13. The browser receives the response, and parses the HTML in the response.
14. A DOM tree is built out of the HTML.
15. New requests are made to the server for each new resource that is found in the HTML source (typically images, style sheets, and JavaScript files). Go back to step 3 and repeat for each resource.
16. Stylesheets are parsed, and the rendering information in each gets attached to the matching node in the DOM tree.
17. Javascript is parsed and executed, and DOM nodes are moved and style information is updated accordingly
18. The browser renders the page on the screen according to the DOM tree and the style information for each node.
19. You see the page on the screen.

Sources

• Friendly Bit