space in flex container code example

Example 1: add space between all html elements flex

/* There is now a `gap` CSS property
** This works for <flex>, <grid>, and <multicolumn> layouts
** #box{} would lay out all it's children with 10px spacing between 
** different rows and 20px betwen different columns
*/
#box {
  display: flex;
  flex-wrap: wrap;
  gap: 10px 20px; /* row-gap [column-gap]*/
}

/* Supported by most modern browesers, Safari excluded. */

Example 2: proper way to make flexbox spacing

/**
Flexbox doesn't have collapsing margins.
Flexbox doesn't have anything akin to border-spacing for tables (except for CSS property gap which isn't supported in Safari, caniuse)
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.
**/
.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>