<!DOCTYPE html>
<html>
<head>
<link rel="author" title="Sammy Gill" href="mailto:sammy.gill@apple.com">
<link rel="help" href="https://drafts.csswg.org/css-box-4/#margin-trim-block">
<meta name="assert" content="item with border should prevent trimming within itself">
<style>
container {
    display: block;
    margin-trim: block;
    border: 1px solid black;
    margin-block-start: 10px;
}
item {
    display: block;
    inline-size: 50px;
    block-size: 50px;
    background-color: green;
}
.collapsed {
  