CSS Fundamentals

CSS (Cascading Style Sheets) controls how HTML elements look. It separates presentation from structure — HTML defines content, CSS defines appearance.

What is CSS?

HTML = Structure (what things are)
CSS = Styling (how things look)
JavaScript = Behavior (what things do)

CSS Syntax

Every CSS rule has a selector and declarations:

selector {
  property: value;
}

Example:

h1 {
  color: blue;
  font-size: 24px;
}

h1 — Selector (targets the element)
color — Property (what to change)
blue — Value (what to change it to)

Three Ways to Add CSS

1. External Stylesheet (Recommended)

Link a separate CSS file:

<head>
  <link rel="stylesheet" href="styles.css" />
</head>

/* styles.css */
h1 {
  color: blue;
}

Benefits:

Single file for entire site
Cached by browser (faster)
Easy to maintain

2. Internal Style Block

Add CSS in the <head>:

<head>
  <style>
    h1 {
      color: blue;
    }
  </style>
</head>

Use for:

Single-page sites
Page-specific styles

3. Inline Styles

Add CSS directly to elements:

<h1 style="color: blue;">Hello</h1>

Avoid when possible:

Hard to maintain
Overrides other styles
Mixes structure and presentation

Comments

Add comments to explain your code:

/* This is a CSS comment */

h1 {
  /* Blue color for headings */
  color: blue;
}

Basic Selectors

Element Selector

Targets all elements of a type:

p {
  color: black;
}

h1 {
  font-size: 32px;
}

Class Selector

Targets elements with a class attribute:

<p class="highlight">Yellow background</p>
<p>Normal paragraph</p>

.highlight {
  background-color: yellow;
}

Note: Classes start with .

ID Selector

Targets a single element with an ID:

<div id="header">Header content</div>

#header {
  background-color: #333;
}

Note: IDs start with #

Properties and Values

Common Properties

/* Text */
h1 {
  color: blue;
  font-size: 24px;
  font-weight: bold;
  text-align: center;
}

/* Background */
body {
  background-color: #f5f5f5;
  background-image: url('bg.jpg');
}

/* Box Model */
div {
  margin: 10px;
  padding: 20px;
  border: 1px solid black;
}

/* Width/Height */
img {
  width: 100%;
  height: auto;
}

Value Types

Colors

/* Named colors */
color: red;

/* Hexadecimal */
color: #ff0000;
color: #f00;

/* RGB */
color: rgb(255, 0, 0);

/* RGBA (with opacity) */
color: rgba(255, 0, 0, 0.5);

/* HSL */
color: hsl(0, 100%, 50%);

Lengths

/* Pixels */
font-size: 16px;

/* Em (relative to parent) */
font-size: 1.5em;

/* Rem (relative to root) */
font-size: 1.5rem;

/* Percentage */
width: 50%;

/* Viewport units */
width: 100vw;
height: 100vh;

Complete Example

Here’s a complete CSS file:

/* styles.css */

/* Reset some defaults */
* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

/* Body styling */
body {
  font-family: Arial, sans-serif;
  line-height: 1.6;
  color: #333;
  background-color: #f5f5f5;
}

/* Headings */
h1 {
  font-size: 2.5rem;
  color: #2c3e50;
  margin-bottom: 1rem;
}

h2 {
  font-size: 2rem;
  color: #34495e;
  margin-bottom: 0.75rem;
}

/* Paragraphs */
p {
  font-size: 1rem;
  margin-bottom: 1rem;
  line-height: 1.8;
}

/* Links */
a {
  color: #3498db;
  text-decoration: none;
}

a:hover {
  color: #2980b9;
  text-decoration: underline;
}

/* Buttons */
.button {
  display: inline-block;
  padding: 10px 20px;
  background-color: #3498db;
  color: white;
  border: none;
  border-radius: 5px;
  cursor: pointer;
}

.button:hover {
  background-color: #2980b9;
}

/* Container */
.container {
  max-width: 1200px;
  margin: 0 auto;
  padding: 20px;
}

Complete HTML with CSS

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>CSS Fundamentals</title>
    <link rel="stylesheet" href="styles.css" />
  </head>
  <body>
    <div class="container">
      <h1>CSS Fundamentals</h1>
      <p>
        CSS controls how HTML elements look.
        <a href="#">Learn more</a>
      </p>
      <button class="button">Click Me</button>
    </div>
  </body>
</html>

Best Practices

Use external stylesheets — Easier to maintain
Use classes over IDs — More reusable
Use meaningful names — .nav-item not .blue
Keep it organized — Group related styles
Use comments — Explain complex styles

Common Mistakes

Missing semicolon — Each declaration needs ;
Missing curly braces — Properties must be inside {}
Using IDs for styling — Use classes instead
Inline styles everywhere — Hard to maintain
Not using box-sizing — Causes layout issues