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+<!DOCTYPE html>
+<!-- Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/ -->
+<html>
+  <meta charset="utf-8">
+  <title>CSS Flexbox Test Reference: Fragmentation of single-line row flex container</title>
+  <link rel="author" title="Ting-Yu Lin" href="mailto:tlin@mozilla.com">
+  <link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+
+  <style>
+  html {
+    writing-mode: vertical-rl;
+  }
+  .multicol {
+    block-size: 50px;
+    inline-size: 320px;
+    column-count: 3;
+    column-fill: auto;
+    column-gap: 10px;
+    border: 5px solid orange;
+    margin-block-end: 5px; /* Just to separate each sub-testcase visually. */
+    padding-block: 15px;
+  }
+  .flexContainer {
+    background: gray;
+    /* border-width is 0 by default; some sub-testcases will increase it. */
+    border: 0 solid white;
+    block-size: 100px;
+    position: relative;
+  }
+  .gap {
+    inline-size: 4px;
+    block-size: 100%;
+  }
+
+  /* Note: flex items are in visual order as "CMYK": cyan, magenta, yellow, black. */
+  header, nav, article, footer {
+    inline-size: 25px;
+  }
+  header {
+    background: cyan;
+    block-size: 100px;
+    position: absolute;
+    inset-block-start: 0px;
+  }
+  nav {
+    background: magenta;
+    block-size: 25px;
+    position: absolute;
+    inset-block-start: 0px;
+    inset-inline-start: 25px;
+  }
+  article {
+    background: yellow;
+    block-size: 50px;
+    position: absolute;
+    inset-block-start: 25px;
+    inset-inline-start: 50px;
+  }
+  footer {
+    background: black;
+    block-size: 75px;
+    position: absolute;
+    inset-block-start: 25px;
+    inset-inline-start: 75px;
+  }
+  </style>
+
+  <body>
+    <!-- Test a container with margin-block-start. -->
+    <div class="multicol">
+      <div class="flexContainer" style="margin-block-start: 25px">
+        <header></header><nav></nav><article></article><footer></footer>
+      </div>
+    </div>
+
+    <!-- Test a container not at the top of the column/page. -->
+    <div class="multicol">
+      <div style="block-size: 25px"></div>
+      <div class="flexContainer">
+        <header></header><nav></nav><article></article><footer></footer>
+      </div>
+    </div>
+
+    <!-- Test a container with border, padding, and column-gap. -->
+    <div class="multicol">
+      <div class="flexContainer border-padding">
+        <header style="inline-size: 22px"></header>
+        <nav style="inline-size: 22px; inset-inline-start: 26px"></nav>
+        <article style="inline-size: 22px; inset-block-start: 25px; inset-inline-start: 52px;"></article>
+        <footer style="inline-size: 22px; inset-block-start: 25px; inset-inline-start: 78px;"></footer>
+      </div>
+    </div>
+  </body>
+</html>