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HTML 

              
                <h2>규칙</h2>
<p>블록의 위아래 여백은 각 여백중 더 큰 것으로 결합(병합)됩니다. 이를 마진 병합(Margin Collapsing)이라고 합니다. float되거나 absolute로 배치된 요소의 마진은 절대 병합되지 않습니다.</p>
<p>참고: <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_box_model/Mastering_margin_collapsing">Mastering margin collapsing(MDN)</a></p>
<h2>위아래 마진은 병합된다</h2>
<p>👇뒤의 요소가 float 요소 뒤에서 clear 속성을 사용하지 않은 경우</p>
<div class="test-block  flex" style="margin-bottom: 50px; background-color: gold;">
  margin-bottom: 50px
</div>
<div class="test-block  flex" style="margin-top: 50px; background-color: skyblue;">
  margin-top: 50px
</div>

<p>👇뒤의 요소가 float 요소 뒤에서 clear 속성을 사용한 경우는 예외</p>
<div class="test-block  flex" style="margin-bottom: 50px; background-color: gold;">
  margin-bottom: 50px
</div>
<div class="float" style="float: left; height: 30px; background-color: lightgray; padding: 5px;">float: left</div>
<div class="test-block  flex  clear" style="margin-top: 50px; background-color: skyblue;">
  margin-top: 50px
</div>

<p>👇뒤의 요소가 float 요소 뒤에서 clear 속성을 사용 안 한 경우는 마진 병합이 작동</p>
<div class="test-block  flex" style="margin-bottom: 50px; background-color: gold;">
  margin-bottom: 50px
</div>
<div class="float" style="float: left; height: 30px; background-color: lightgray; padding: 5px;">float: left</div>
<div class="test-block  flex" style="margin-top: 50px; background-color: skyblue;">
  margin-top: 50px
</div>


<h2>특정 조건에서 부모 자식간 마진은 병합된다</h2>
<p style="font-size: 14px;">👇자식의 margin-top에서 부모의 margin-top을 분리할 요소가 부모에 없는 경우(border, padding, inline 부분, <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_display/Block_formatting_context">블록 형식 맥락</a>이나 clear)</p>
<p>이 아래는 자식 마진이 적용돼 마진이 50px 있다</p>
<div style="background-color: gold;">
  <div style="margin: 50px 10px 0; background-color: skyblue;">
    margin-top: 50px
  </div>
  부모
</div>


<p style="font-size: 14px;">👇자식의 margin-top에서 부모의 margin-top을 분리할 border가 부모에 <strong>있는</strong> 경우</p>
<div style="background-color: gold; border: 1px solid black;">
  <div style="margin: 50px 10px 0; background-color: skyblue;">
    margin-top: 50px
  </div>
  부모(border: 1px solid black)
</div>



<p style="font-size: 14px;">👇부모에 자식의 margin-bottom을 분리할 border, padding, inline 내용, 높이(height, min-height)가 없는 경우</p>
<div style="background-color: gold;">
  부모
  <div style="margin: 0 10px 50px; background-color: skyblue;">
    margin-bottom: 50px
  </div>
</div>
<p>이 위로는 자식 마진이 적용돼 마진이 50px 있다</p>


<p style="font-size: 14px;">👇부모에 자식의 margin-bottom을 분리할 height가 있는 경우</p>
<div style="background-color: gold; height: 130px;">
  부모(height: 130px;)
  <div style="margin: 0 10px 50px; background-color: skyblue;">
    margin-bottom: 50px
  </div>
</div>
<p style="margin: 0;">이 위로는 자식 마진이 작동하지 않아 딱 붙는다.</p>



<h2>border, padding, inline 내용, 높이<sub>(height, min-height)</sub>가 없는 빈 블록의 상하 마진은 스스로 병합된다</h2>
<p>블록 위 (위아래 간격은 30px, 블록에는 margin-top: 30px; margin-bottom: 20px 설정)</p>
<div style="margin-top: 30px; margin-bottom: 20px;"></div>
<p>블록 아래</p>

<h2>좌우 마진은 병합되지 않는다</h2>
<div>
  <div class="test-block  inline-flex  test-block--1">margin-right: 50px</div>
  <div class="test-block  inline-flex  test-block--2">margin-left: 50px;</div>
</div>
!

CSS 

              
                html {
  font-family: Noto Sans, sans-serif;
  max-width: 800px;
  line-height: 1.7;
  box-sizing: border-box;
}

h2 {
  line-height: 1.3;
}

h2:not(:first-child) {
  margin-top: 2em;
  padding-top: 1em;
  border-top: 1px dashed #999;
}

.test-block {
  height: 100px;
  width: 200px;
  align-items: center;
  justify-content: center;
  text-align: center;
  font-family: sans-serif;
}

.flex {
  display: flex;
}

.inline-flex {
  display: inline-flex;
}

.clear {
  clear: both;
}

.test-block--1 {
  background-color: gold;
  color: black;
  margin-right: 50px;
}

.test-block--2 {
  background-color: skyblue;
  color: black;
  margin-left: 50px;
}
!

JS
!
999px

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