margin-trim

none

Margins are not trimmed by the container.

in-flow

For in-flow boxes contained by this box, block-axis margins adjacent to the box's edges are truncated to zero.

It also truncates any margins collapsed with such a margin.

all

Trims the margins of in-flow boxes and floats whose margins coincide with the container's content edge.

margin-trim = 
  none                                                |
  block                                               |
  inline                                              |
  [ block-start || inline-start || block-end || inline-end ]  

Once support is implemented for this property, it will probably work like so:

When you've got a container with some inline children and you want to put a margin between each child but not have it interfere with the spacing at the end of the row, you might do something like this:

article {
  background-color: red;
  margin: 20px;
  padding: 20px;
  display: inline-block;
}

article > span {
  background-color: black;
  color: white;
  text-align: center;
  padding: 10px;
  margin-right: 20px;
}

The problem here is that you'd end up with 20px too much spacing at the right of the row, so you'd maybe do this to fix it:

span:last-child {
  margin-right: 0;
}

It is a pain having to write another rule to achieve this, and it is also not very flexible. Instead, margin-trim could fix it:

article {
  margin-trim: in-flow;
  /* … */
}

• margin