none

Margins are not trimmed by the container.

block

Margins provided to the block children where they adjoin the container's edges are trimmed to zero without affecting the margins provided to the container.

block-start

Margin of the first block child with the container's edge is trimmed to zero.

block-end

Margin of last block child with the container's edge is trimmed to zero.

inline

Margins provided to the inline children where they adjoin the container's edges are trimmed to zero, without affecting the spacing at the beginning and end of the row.

inline-start

Margin between the container's edge and the first inline child is trimmed to zero.

inline-end

Margin between the container's edge and the last inline child is trimmed to zero.

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;
  margin-left: 30px;
}

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;
  margin-left: 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: inline-end;
  /* … */
}

Similarly, to remove left margin with the container's edge:

article {
  margin-trim: inline-start;
  /* … */
}

• margin