CSS display block property margin issue

Question

Sometimes I use margin on div (display: block) and doesn't work properly, but with inline-block works, my doubt is why? Should the margin in this case work in the same way?

I know that I can use .box margin-top, .box + box margin-top, etc... but this is not the question.

.box {
  border: 1px solid #500;
}
.block-div-margin {
  margin: 10px 0;
  display: block;
  /*default div state*/
}
.inline-block-div-margin {
  display: inline-block;
  margin: 10px 0;
}
.my-div {
  padding: 20px;
  background: #EAEAEA;
  border-bottom: 1px solid #999;
}
.my-other-div {
  background: #D3E1E1;
  padding: 0 20px 20px 20px;
}
.my-other-div-content {
  margin-top: 20px;
}
.my-div-inline-block {
  display: inline-block;
  width: 100%;
  box-sizing: border-box;
}

<h2>Example 1</h2>
<h3>block-div-margin</h3>
<div class="container">
  <div class="box">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Quas, nobis.</div>
  <div class="block-div-margin"></div>
  <div class="box">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Doloremque, delectus!</div>
  <div class="block-div-margin"></div>
  <div class="box">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Aspernatur, quasi.</div>
  <div class="block-div-margin"></div>
  <div class="box">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Porro, id!</div>
</div>

<h3>inline-block-div-margin</h3>
<div class="container">
  <div class="box">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Quas, nobis.</div>
  <div class="inline-block-div-margin"></div>
  <div class="box">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Doloremque, delectus!</div>
  <div class="inline-block-div-margin"></div>
  <div class="box">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Aspernatur, quasi.</div>
  <div class="inline-block-div-margin"></div>
  <div class="box">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Porro, id!</div>
</div>

<h2>Example 2</h2>
<h3>This is more common issue that I see ever</h3>
<div class="my-div">
  This is my-div content
</div>
<div class="my-other-div">
  <div class="my-other-div-content">
    This is my-other-div-content
  </div>
</div>

<h3>With div inline-block</h3>
<div class="my-div">
  This is my-div content
</div>
<div class="my-other-div my-div-inline-block">
  <div class="my-other-div-content">
    This is my-other-div-content
  </div>
</div>

Even with the element inside another element, when adding margin, it is as if I have added the margin in the parent element and not in the child.

Answer 1

It's called collapsing margins, and that doesn't happen on inline blocks.

8.3.1 Collapsing margins

Margins of inline-block boxes do not collapse (not even with their in-flow children).

Follow this post for more ways to avoid that.