Learn the CSS Box Model by Building a Rothko Painting - Step 21

Tell us what’s happening:
Describe your issue in detail here.
I do not understand the margin part, I think when we add top-margin:20px to the div class=“one”, this div one should be below the div class=“canvas” by 20px, but instead of that, the top margin of the div one is outside the div canvas. Can you explain to me why? Thank you.

  **Your code so far**
/* file: styles.css */
.canvas {
width: 500px;
height: 600px;
background-color: #4d0f00;


.frame {
border: 50px solid red;
width: 500px;
padding: 50px;
margin: 20px auto;

.one {
width: 425px;
height: 200px;
background-color: #efb762;
margin: 20px auto;
/* file: index.html */
<!DOCTYPE html>
<html lang="en">
  <meta charset="UTF-8">
  <title>Rothko Painting</title>
  <link href="./styles.css" rel="stylesheet">
  <div class="frame">
    <div class="canvas">
      <div class="one"></div>
  **Your browser information:**

User Agent is: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/ Safari/537.36

Challenge: Learn the CSS Box Model by Building a Rothko Painting - Step 21

Link to the challenge:

look at HTML tho.
.one is inside .canvas, right?

Someone please explain this.

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Thank you.

Thats right, but the question is:

WHY the top-margin of div class=“one” is outside the div class=“canvas”?

Finally found the explanation. :slight_smile:

Top-margin of .one element COLLAPSE with top-margin of .canvas element, which is called Collapsing margin.

Relevant explanation from the W3C spec:

8.3.1 Collapsing margins

In CSS, the adjoining margins of two or more boxes (which might or might not be siblings) can combine to form a single margin. Margins that combine this way are said to collapse, and the resulting combined margin is called a collapsed margin.

:star: Adjoining vertical margins collapse :star:

Two margins are adjoining if and only if:

  • both belong to in-flow block-level boxes that participate in the same block formatting context
  • no line boxes, no clearance, no padding and no border separate them (Note that certain zero-height line boxes (see 9.4.2) are ignored for this purpose.)
  • both belong to vertically-adjacent box edges, i.e. form one of the following pairs:
    • top margin of a box and top margin of its first in-flow child

The left and right margins behave as you expect because:

:star: Horizontal margins never collapse. :star:

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