The difference between flex-basis auto and 0 (zero)

“0” and “auto” flex-basis will be different in most if not all situations: numeric values are interpreted as specific widths, so zero would be the same as specifying "width: 0" — and thus will collapse the element to its smallest possible width given the content, or to zero itself if its overflow is hidden or the element is directly sizable (an image for example.)

.f {display:flex}
.f div {border:1px solid}
.zero {flex-basis: 0}
.auto {flex-basis: auto}

<div class="f">
  <div class="zero">This is flex-basis zero</div>
</div>

<div class="f">
  <div class="auto">This is flex-basis auto</div>
</div>