How to justify a single flexbox item (override justify-content)

There doesn't seem to be justify-self, but you can achieve similar result setting appropriate margin to auto¹. E. g. for flex-direction: row (default) you should set margin-right: auto to align the child to the left.

.container {
  height: 100px;
  border: solid 10px skyblue;
  
  display: flex;
  justify-content: flex-end;
}
.block {
  width: 50px;
  background: tomato;
}
.justify-start {
  margin-right: auto;
}

<div class="container">
  <div class="block justify-start"></div>
  <div class="block"></div>
</div>

¹ This behaviour is defined by the Flexbox spec.

AFAIK there is no property for that in the specs, but here is a trick I’ve been using: set the container element ( the one with display:flex ) to justify-content:space-around Then add an extra element between the first and second item and set it to flex-grow:10 (or some other value that works with your setup)

Edit: if the items are tightly aligned it's a good idea to add flex-shrink: 10; to the extra element as well, so the layout will be properly responsive on smaller devices.