You don't need floats. In fact, they're useless because floats are ignored in flexbox.
You also don't need CSS positioning.
There are several flex methods available. auto
margins have been mentioned in another answer.
Here are two other options:
justify-content: space-between
and the order
property.justify-content: space-between
and reverse the order of the divs..parent {_x000D_
display: flex;_x000D_
justify-content: space-between;_x000D_
}_x000D_
_x000D_
.parent:first-of-type > div:last-child { order: -1; }_x000D_
_x000D_
p { background-color: #ddd;}
_x000D_
<p>Method 1: Use <code>justify-content: space-between</code> and <code>order-1</code></p>_x000D_
_x000D_
<div class="parent">_x000D_
<div class="child" style="float:right"> Ignore parent? </div>_x000D_
<div>another child </div>_x000D_
</div>_x000D_
_x000D_
<hr>_x000D_
_x000D_
<p>Method 2: Use <code>justify-content: space-between</code> and reverse the order of _x000D_
divs in the mark-up</p>_x000D_
_x000D_
<div class="parent">_x000D_
<div>another child </div>_x000D_
<div class="child" style="float:right"> Ignore parent? </div>_x000D_
</div>
_x000D_