I don't think what I'm trying to do is possible.
<div class="row">
<div class="item"></div>
<div class="item"></div>
<div class="item"></div>
</div>
<div class="row">
<div class="item"></div>
<div class="item"></div>
</div>
<div class="row">
<div class="item"></div>
</div>
I want all the .items to take up the maximum, identical horizontal space, that is (about) 33% in the 1st one, 50% in the second one, and 100% in the last.
The idea here is to not set the size of the .items (and avoiding setting ids to elements) for maximum flexibility and minimum error surface ; If this is impossible with FlexBox but doable using another strategy, please share it.