<div>
<span>スパン</span>
<span>スパン</span>
</div>
<div>
<span>スパン</span>
<span class="flex_span">スパン</span>
</div>

.flex_span {
  display:flex;
}

div{
  border: 2px solid #a88a74;
  border-radius: 5px;
  background-color: #f9ebb8;
  width: 400px;
  padding: 5px;
  margin: 10px
}