Better way to set distance between flexbox items

CSS gap property:

There is a new gap CSS property for multi-column, flexbox, and grid layouts that works in newer browsers now! (See Can I use link 1; link 2). It is shorthand for row-gap and column-gap.

#box {
  display: flex;
  gap: 10px;
}

CSS row-gap property:

The row-gap CSS property for both flexbox and grid layouts allows you to create a gap between rows.

#box {
   display: flex;
   row-gap: 10px;
}

CSS column-gap property:

The column-gap CSS property for multi-column, flexbox and grid layouts allows you to create a gap between columns.

#box {
  display: flex;
  column-gap: 10px;
}

Example:

#box {
  display: flex;
  flex-wrap: wrap;
  width: 200px;
  background-color: red;
  gap: 10px;
}
.item {
  background: gray;
  width: 50px;
  height: 50px;
  border: 1px black solid;
}

<div id='box'>
  <div class='item'></div>
  <div class='item'></div>
  <div class='item'></div>
  <div class='item'></div>
</div>

• Flexbox doesn't have collapsing margins.
• Flexbox doesn't have anything akin to border-spacing for tables (edit: CSS property gap fulfills this role in newer browsers, Can I use)

Therefore achieving what you are asking for is a bit more difficult.

In my experience, the "cleanest" way that doesn't use :first-child/:last-child and works without any modification on flex-wrap:wrap is to set padding:5px on the container and margin:5px on the children. That will produce a 10px gap between each child and between each child and their parent.

Demo

.upper {
  margin: 30px;
  display: flex;
  flex-direction: row;
  width: 300px;
  height: 80px;
  border: 1px red solid;

  padding: 5px; /* this */
}

.upper > div {
  flex: 1 1 auto;
  border: 1px red solid;
  text-align: center;

  margin: 5px;  /* and that, will result in a 10px gap */
}

.upper.mc /* multicol test */ {
  flex-direction: column;
  flex-wrap: wrap;
  width: 200px;
  height: 200px;
}

<div class="upper">
  <div>aaa<br/>aaa</div>
  <div>aaa</div>
  <div>aaa<br/>aaa</div>
  <div>aaa<br/>aaa<br/>aaa</div>
  <div>aaa</div>
  <div>aaa</div>
</div>

<div class="upper mc">
  <div>aaa<br/>aaa</div>
  <div>aaa</div>
  <div>aaa<br/>aaa</div>
  <div>aaa<br/>aaa<br/>aaa</div>
  <div>aaa</div>
  <div>aaa</div>
</div>