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diff --git a/layout/style/test/test_pixel_lengths.html b/layout/style/test/test_pixel_lengths.html
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+++ b/layout/style/test/test_pixel_lengths.html
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+<!DOCTYPE HTML>
+<html>
+<head>
+  <title>Test that pixel lengths don't change based on DPI</title>
+  <script type="text/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
+</head>
+<body>
+<div id="display">
+
+<div id="pt" style="width:90pt; height:90pt; background:lime;">pt</div>
+<div id="pc" style="width:5pc; height:5pc; background:yellow;">pc</div>
+<div id="mm" style="width:25.4mm; height:25.4mm; background:orange;">mm</div>
+<div id="cm" style="width:2.54cm; height:2.54cm; background:purple;">cm</div>
+<div id="in" style="width:1in; height:1in; background:magenta;">in</div>
+<div id="q" style="width:101.6q; height:101.6q; background:blue;">q</div>
+
+<div id="mozmm" style="width:25.4mozmm; height:25.4mozmm; background:cyan;">mozmm</div>
+
+</div>
+<pre id="test">
+<script class="testbody" type="text/javascript">
+
+var oldDPI = SpecialPowers.getIntPref("layout.css.dpi");
+var dpi = oldDPI;
+
+function check(id, val) {
+  var e = document.getElementById(id);
+  is(Math.round(e.getBoundingClientRect().width), Math.round(val),
+     "Checking width in " + id + " at " + dpi + " DPI");
+  is(Math.round(e.getBoundingClientRect().height), Math.round(val),
+     "Checking height in " + id + " at " + dpi + " DPI");
+}
+
+function checkPixelRelativeUnits() {
+  check("pt", 120);
+  check("pc", 80);
+  check("mm", 96);
+  check("cm", 96);
+  check("in", 96);
+  check("q", 96);
+}
+
+checkPixelRelativeUnits();
+
+SimpleTest.waitForExplicitFinish();
+
+SpecialPowers.pushPrefEnv({'set': [['layout.css.dpi', dpi=96]]}, test1);
+
+var mozmmSize;
+function test1() {
+  var mozmm = document.getElementById("mozmm");
+  mozmmSize = mozmm.getBoundingClientRect().width;
+  is(Math.round(mozmmSize), Math.round(mozmm.getBoundingClientRect().height),
+     "mozmm div should be square");
+
+  checkPixelRelativeUnits();
+
+  SpecialPowers.pushPrefEnv({'set': [['layout.css.dpi', dpi=192]]}, test2);
+}
+
+function test2() {
+  // At 192 dpi, a one-inch box should be twice the number of device pixels,
+  // and since we haven't changed the device-pixels-per-CSS-pixel ratio, the
+  // mozmm box should be twice the size in CSS pixels.
+  check("mozmm", mozmmSize*2);
+  checkPixelRelativeUnits();
+
+  SimpleTest.finish();
+}
+
+</script>
+</pre>
+</body>
+</html>