CSS Fundamentals for Beginners: Complete Student Guide

<!-- In the <head> of your HTML -->
<link rel="stylesheet" href="styles.css">

/* styles.css — all your CSS goes here */
h1 {
  color: #10b981;
  font-size: 2.5rem;
  text-align: center;
}

p {
  font-size: 1.1rem;
  line-height: 1.8;
  color: #334155;
}

<head>
  <style>
    body {
      background-color: #f8fafc;
      font-family: system-ui, sans-serif;
    }
    h1 {
      color: #10b981;
    }
  </style>
</head>

<!-- Inline style — use sparingly! -->
<h1 style="color: #10b981; font-size: 2rem;">Hello World</h1>

/* 1. Type/Element Selector — targets all elements of a type */
p {
  color: #334155;
}

/* 2. Class Selector — targets elements with a specific class */
.highlight {
  background-color: #fef3c7;
  padding: 0.5rem;
}

/* 3. ID Selector — targets a unique element (use sparingly!) */
#main-title {
  font-size: 3rem;
  color: #059669;
}

/* 4. Descendant Selector — targets elements inside another element */
.card p {
  margin-bottom: 1rem;
}

/* 5. Child Selector — targets direct children only */
.menu > li {
  display: inline-block;
}

/* 6. Pseudo-Class — targets elements in a specific state */
a:hover {
  color: #34d399;
  text-decoration: underline;
}

/* 7. Pseudo-Element — targets part of an element */
p::first-line {
  font-weight: bold;
}

/* 8. Attribute Selector — targets elements with specific attributes */
[type="text"] {
  border: 2px solid #cbd5e1;
}

/* 9. Multiple Selectors — target multiple elements at once */
h1, h2, h3 {
  font-weight: 700;
  line-height: 1.2;
}

/* Named Colors */
.text-primary {
  color: blue;
}

/* Hex Colors (6-digit) */
.text-hex {
  color: #10b981;
}

/* RGB (Red, Green, Blue) */
.text-rgb {
  color: rgb(16, 185, 129);
}

/* HSL (Hue, Saturation, Lightness) */
.text-hsl {
  color: hsl(160, 84%, 39%);
}

/* RGBA (with Alpha/Opacity) */
.bg-transparent {
  background-color: rgba(16, 185, 129, 0.2);
}

/* CSS Variables / Custom Properties */
:root {
  --primary-color: #10b981;
  --secondary-color: #ec4899;
}

.using-variables {
  color: var(--primary-color);
}

/* Base text styles */
body {
  font-family: 'Inter', system-ui, -apple-system, sans-serif;
  font-size: 1rem;
  line-height: 1.7;
  color: #1e293b;
}

/* Headings */
h1 {
  font-size: 2.5rem;
  font-weight: 800;
  letter-spacing: -0.025em;
}

h2 {
  font-size: 2rem;
  font-weight: 700;
}

/* Text alignment and decoration */
.text-center {
  text-align: center;
}
.text-uppercase {
  text-transform: uppercase;
}
.text-decoration {
  text-decoration: underline wavy #34d399;
}

/* Google Fonts import (in HTML head) */
/* <link href="https://fonts.googleapis.com/css2?family=Inter:opsz@14..32&display=swap" rel="stylesheet"> */

/* The box model consists of:
   content → padding → border → margin */

.box {
  /* Content area */
  width: 300px;
  height: 200px;

  /* Padding — space inside the border */
  padding: 20px;

  /* Border — the edge of the box */
  border: 2px solid #10b981;

  /* Margin — space outside the border */
  margin: 15px;

  /* Background */
  background-color: #f8fafc;
}

/* Shorthand properties */
.shorthand {
  /* Top Right Bottom Left */
  padding: 10px 20px 10px 20px;
  margin: 0 auto; /* Centering horizontally */
  border: 3px solid #ec4899;
}

/* Box-sizing: border-box (recommended) */
* {
  box-sizing: border-box;
}

/* With border-box, padding and border are included in the width/height */
.with-border-box {
  width: 300px;
  padding: 20px;
  border: 2px solid #10b981;
  /* Total width = 300px (content + padding + border) */
}

/* Container — parent element */
.flex-container {
  display: flex;
  flex-direction: row; /* row (default), column, row-reverse, column-reverse */
  justify-content: center; /* main-axis alignment */
  align-items: center; /* cross-axis alignment */
  flex-wrap: wrap; /* wrap items to next line */
  gap: 1rem; /* spacing between items */
}

/* Justify-content options */
.flex-start { justify-content: flex-start; }
.flex-end   { justify-content: flex-end; }
.center     { justify-content: center; }
.space-between { justify-content: space-between; }
.space-around { justify-content: space-around; }
.space-evenly { justify-content: space-evenly; }

/* Align-items options */
.items-start { align-items: flex-start; }
.items-end   { align-items: flex-end; }
.items-center { align-items: center; }
.items-stretch { align-items: stretch; }

/* Child items — individual elements */
.flex-item {
  flex: 1; /* grow equally */
  flex-grow: 1; /* can grow to fill space */
  flex-shrink: 0; /* don't shrink */
  flex-basis: 200px; /* starting size */
  align-self: center; /* overrides align-items */
}

/* Common Flexbox patterns */
.nav-bar {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 1rem 2rem;
}

.card-grid {
  display: flex;
  flex-wrap: wrap;
  gap: 1.5rem;
}

.card-grid .card {
  flex: 1 1 250px; /* grow, shrink, basis */
}

/* Container — parent element */
.grid-container {
  display: grid;
  grid-template-columns: repeat(3, 1fr); /* 3 equal columns */
  grid-template-rows: auto 1fr auto; /* header, main, footer */
  gap: 1.5rem;
}

/* Column templates */
.two-columns {
  grid-template-columns: 1fr 2fr; /* 1:2 ratio */
}
.three-columns {
  grid-template-columns: repeat(3, 1fr);
}
.responsive-grid {
  grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
}

/* Grid item placement */
.header {
  grid-column: 1 / -1; /* span all columns */
}
.sidebar {
  grid-row: 2 / 4; /* span rows 2-3 */
}
.main-content {
  grid-column: 2 / 4; /* columns 2-3 */
}

/* Common page layout with Grid */
.page-layout {
  display: grid;
  grid-template-areas: 
    "header header header"
    "sidebar main main"
    "footer footer footer";
  grid-template-columns: 250px 1fr 1fr;
  grid-template-rows: auto 1fr auto;
  min-height: 100vh;
  gap: 1rem;
}

.page-layout .header { grid-area: header; }
.page-layout .sidebar { grid-area: sidebar; }
.page-layout .main { grid-area: main; }
.page-layout .footer { grid-area: footer; }

/* Default styles (mobile-first) */
.container {
  display: block;
  padding: 1rem;
}

.grid {
  display: grid;
  grid-template-columns: 1fr; /* Single column on mobile */
  gap: 1rem;
}

/* Tablet: 768px and up */
@media (min-width: 768px) {
  .grid {
    grid-template-columns: repeat(2, 1fr);
  }
  .container {
    padding: 2rem;
  }
}

/* Desktop: 1024px and up */
@media (min-width: 1024px) {
  .grid {
    grid-template-columns: repeat(3, 1fr);
  }
  .container {
    max-width: 1200px;
    margin: 0 auto;
  }
}

/* Desktop: 1440px and up (large screens) */
@media (min-width: 1440px) {
  .grid {
    grid-template-columns: repeat(4, 1fr);
  }
}

/* Mobile-first approach: start with mobile, add styles for larger screens */

/* Common responsive patterns */
.responsive-nav {
  display: flex;
  flex-direction: column;
  gap: 0.5rem;
}

@media (min-width: 768px) {
  .responsive-nav {
    flex-direction: row;
    gap: 2rem;
  }
}

/* Transitions — smooth changes between states */
.button {
  background-color: #10b981;
  color: white;
  padding: 0.75rem 1.5rem;
  border-radius: 0.5rem;
  transition: all 0.3s ease;
}

.button:hover {
  background-color: #059669;
  transform: scale(1.05);
  box-shadow: 0 4px 14px rgba(16, 185, 129, 0.4);
}

/* Individual transition properties */
.card {
  transition-property: transform, box-shadow;
  transition-duration: 0.3s;
  transition-timing-function: ease-out;
}

/* Keyframe animations */
@keyframes fadeIn {
  from {
    opacity: 0;
    transform: translateY(20px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

@keyframes pulse {
  0% {
    transform: scale(1);
  }
  50% {
    transform: scale(1.05);
  }
  100% {
    transform: scale(1);
  }
}

@keyframes slideIn {
  0% {
    transform: translateX(-100%);
  }
  100% {
    transform: translateX(0);
  }
}

/* Applying animations */
.animate-fade {
  animation: fadeIn 0.8s ease-in-out forwards;
}

.animate-pulse {
  animation: pulse 2s ease-in-out infinite;
}

.animate-slide {
  animation: slideIn 0.5s ease-out forwards;
}

/* Animation shorthands */
.custom-animation {
  /* animation: name duration timing-function delay iteration-count direction fill-mode; */
  animation: fadeIn 0.6s ease-out 0.2s 1 normal forwards;
}