Author: GPT-4

Introduction

CSS (Cascading Style Sheets) is an essential part of modern web development, allowing developers to control the appearance and layout of web pages. To understand how browsers parse and render CSS, it is crucial to know about the parsing process and the handling of various @rules. This guide will provide insights into the CSS parsing process in browsers and discuss different @rules, including media queries, animations, keyframes, and more.

CSS Parsing

When a browser receives a CSS file or encounters an embedded style sheet, it must parse the CSS code into a structure that it can understand and use to render a web page. The browser follows the steps below to parse a CSS file:

  1. Tokenization: The browser converts the raw CSS code into a series of tokens. Tokens are the smallest possible units of code that have meaning, such as identifiers, keywords, and punctuation marks.

    body {
      background-color: #f0f0f0;
    }
    

    In this example, some tokens would be 'body', '{', 'background-color', ':', '#f0f0f0', ';', and '}'.

  2. Parsing: The browser takes the tokens and creates a tree-like structure called the CSSOM (CSS Object Model), which represents the style rules and their relationships. The CSSOM is used by the browser to apply the styles to the DOM (Document Object Model) and render the page.

  3. Handling @rules: During the parsing process, the browser encounters various @rules, which are special instructions that affect the interpretation of the stylesheet. Examples of @rules include media queries, animations, and keyframes. The browser handles each @rule differently, depending on its purpose.

CSSOM Structure and Components

The CSSOM (CSS Object Model) is a tree-like structure representing the CSS styles and their relationships. It is used by the browser to apply styles to the DOM (Document Object Model) and render the web page. The main components of the CSSOM include CSSStyleRules, CSSDeclarations, and @rules. Let's take a closer look at these components and their pseudocode representation.

CSSOM Pseudocode

The CSSOM is represented as a tree structure, with the root node being the CSSStyleSheet object. Each CSSStyleSheet has a list of CSSRules, which can include CSSStyleRule(s), @rules, and other rule types. Here's a pseudocode representation of the CSSOM structure:

CSSStyleSheet {
  cssRules: [
    CSSStyleRule,
    CSSMediaRule,
    CSSKeyframesRule,
    ...
  ]
}

CSSStyleRule

A CSSStyleRule represents a single CSS rule, consisting of a selector and a declaration block. The declaration block contains a list of CSSDeclarations, which are individual property-value pairs.

Here's a pseudocode representation of a CSSStyleRule:

CSSStyleRule {
  selectorText: "body",
  style: CSSStyleDeclaration {
    length: 2,
    0: "background-color",
    1: "font-size",
    ...
  }
}

CSSDeclaration

A CSSDeclaration represents a single CSS property-value pair within a declaration block. It has a property name and a value. Here's a pseudocode representation of a CSSDeclaration: