Understanding the working of browsers

Recently I have read about the basic internal working of web browsers. Here is a brief note on it.

Web browsers are the application softwares intended to request web resources from the server and display it on client machine screen. Web resources can be markup documets, pdf or image files. They are located on the internet using URI.

A browser have the following main parts:

  1. User interface window
  2. Web browser engine
  3. Networking
  4. Javascript engine
  5. UI backend
  6. Data storage

1. User Interface Window

The major components of browser interface include:

  • Address bar: to input the web address (URI)
  • Content window: to display the loaded resource
  • Reload and Stop buttons: to re-load the web resource or to stop its loading
  • Back and Forward buttons: to navigate between recently loaded pages
  • Bookmark: option to save the URI for later reference

2. Web Browser Engine (Layout engine or Rendering engine)

It takes marked up content (such as HTML, XML, image files, etc.) and formatting information (such as CSS, XSL, etc.) from the networking layer, parse it and then displays the formatted content on the screen.

Parsing includes tokenization and parse tree (DOM tree and render tree) formation, which is followed by layout and paint events on the content window.

Here is a list of recent browser engines:

  • Blink: Google Chrome, Opera
  • Gecko: Firefox
  • Trident: Internet Explorer
  • Webkit: Safari

Chrome runs multiple instances of rendering engine one for each tab. (Each tab runs a separate process).

3. Networking

Responsible for network calls such as HTTP requests, REST etc.

4. Javascript Engine

It interprets and executes javascript. List of js engines:

  • V8: Google Chrome
  • Spidermonkey and Rhino: Firefox
  • Chakra: Internet Explorer
  • JavaScriptCore: Safari

5. UI Backend

This draws OS specific widgets such as input fields, windows, combo boxes etc.

6. Data Storage

It is a small database saved on client computer which store cache, cookies, etc. Browsers support storage mechanisms such as localStorage, IndexedDB, WebSQL and FileSystem.

Here is an illustration of these processes:

Browser working
Working of browser
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