What Is CSS and How Does It Work?

This guide explains Cascading Style Sheets (CSS), the core technology responsible for the visual presentation of modern websites. You will learn what CSS is, how it interacts with HTML, the fundamental syntax used to write style rules, and why it is essential for modern web development.

Understanding CSS

CSS stands for Cascading Style Sheets. While HTML (HyperText Markup Language) structures the content of a webpage—such as defining headings, paragraphs, and links—CSS controls how those elements look. It determines colors, fonts, layouts, spacing, and animations across different screen sizes. For an extensive collection of documentation and tutorials, you can visit this CSS resource website.

How CSS Works with HTML

A browser renders a webpage by combining HTML structure with CSS styling rules. A CSS rule consists of two main parts: a selector and a declaration block.

Example:

p {
  color: #333333;
  font-size: 16px;
  line-height: 1.5;
}

In this example, the selector p targets all paragraph elements, setting their text color, font size, and line height.

Methods to Apply CSS

CSS can be added to HTML documents in three ways:

  1. External CSS: Written in a separate .css file and linked within the HTML <head> section. This is the most efficient method because a single stylesheet can control the look of an entire website.
  2. Internal CSS: Placed within <style> tags directly inside the <head> section of an individual HTML page. Useful for single-page styling.
  3. Inline CSS: Applied directly to an HTML element using the style attribute. This is generally discouraged for broad design because it mixes content with presentation.

Why CSS Is Essential