Start here / Your roadmap

01 / What is the CSS box model?

Browsers treat each HTML element as a box. That box is made up of content, padding, border, and margin.

02 / Why box-sizing matters

Imagine you set an element’s width to 300 pixels, add 20 pixels of padding on each side, and add a 5-pixel border on each side.

These are illustrations of the two sizing models. The important difference is what the declared width includes.

03 / The one-line fix

A common best practice is to apply box-sizing: border-box to every element, including pseudo-elements.

*,
*::before,
*::after {
  box-sizing: border-box;
}

The * selector means “all elements.” The pseudo-element selectors help ensure generated content follows the same sizing rule.

04 / What is @media screen?

A media query lets you apply CSS only when a condition matches. The screen media type targets screen-based devices and displays. A max-width condition can apply different styles when the viewport becomes narrow.

/* Default: three columns on a wider screen */
.card-grid {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: 20px;
}

/* At 600px wide or narrower, use one column */
@media screen and (max-width: 600px) {
  .card-grid {
    grid-template-columns: 1fr;
  }
}

05 / See a responsive layout

This sample layout starts with three cards. At tablet size it changes to two columns, and on a phone it becomes one column.

Resize your browser window to see this lesson page’s cards reflow. The example above is a visual reference for the assignment you will build in CodePen.

06 / Your CodePen activity

Starter code / paste everything into CodePen’s HTML panel

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>My Responsive Cards</title>
  <style>
    *,
    *::before,
    *::after {
      box-sizing: border-box;
    }

    body {
      margin: 0;
      padding: 24px;
      font-family: Arial, sans-serif;
      background: #f4f1eb;
      color: #182033;
    }

    .page {
      max-width: 1000px;
      margin: 0 auto;
    }

    h1 {
      font-size: clamp(2rem, 5vw, 3.5rem);
      line-height: 1.05;
    }

    .card-grid {
      display: grid;
      grid-template-columns: repeat(3, minmax(0, 1fr));
      gap: 18px;
    }

    .card {
      padding: 22px;
      border: 2px solid #182033;
      background: white;
      min-width: 0;
    }

    .card h2 {
      margin-top: 0;
    }

    /* Medium screens: two columns */
    @media screen and (max-width: 800px) {
      .card-grid {
        grid-template-columns: repeat(2, minmax(0, 1fr));
      }
    }

    /* Small screens: one column */
    @media screen and (max-width: 600px) {
      body {
        padding: 14px;
      }

      .card-grid {
        grid-template-columns: 1fr;
      }

      .card {
        padding: 18px;
      }
    }
  </style>
</head>
<body>
  <main class="page">
    <p>AMDE 155 / Responsive layout practice</p>
    <h1>A Collection of Ideas</h1>
    <p>A short introduction to your collection. Replace this text with your own topic.</p>

    <section class="card-grid">
      <article class="card">
        <h2>Idea One</h2>
        <p>Describe the first item in your collection.</p>
      </article>
      <article class="card">
        <h2>Idea Two</h2>
        <p>Describe the second item in your collection.</p>
      </article>
      <article class="card">
        <h2>Idea Three</h2>
        <p>Describe the third item in your collection.</p>
      </article>
    </section>
  </main>
</body>
</html>

07 / Your checklist

• The entire activity is completed in one CodePen HTML panel.
• The page includes a heading, introduction, and at least three cards.
• The universal box-sizing: border-box rule is included.
• The cards use CSS Grid on wider screens.
• At least two @media screen rules change the layout at different widths.
• The page changes to two columns at medium width and one column on a small screen.
• Text remains readable and elements do not overflow the viewport.
• You customize the content and at least some visual styling.
• You save your Pen and submit its share URL as instructed in class.

Quick reference