diff --git a/layout/style/crashtests/580685.html b/layout/style/crashtests/580685.html
new file mode 100644
index 00000000000..4cb01539350
--- /dev/null
+++ b/layout/style/crashtests/580685.html
@@ -0,0 +1,10 @@
+<html>
+<head></head>
+<body style="outline-offset: 0.1rem; ">
+<script>
+var body = document.body;
+document.removeChild(document.documentElement);
+var compstyle = window.getComputedStyle(body, null).getPropertyValue('outline-offset');
+</script>
+</body>
+</html>
\ No newline at end of file
diff --git a/layout/style/crashtests/crashtests.list b/layout/style/crashtests/crashtests.list
index 18e4a756e44..5725c36198d 100644
--- a/layout/style/crashtests/crashtests.list
+++ b/layout/style/crashtests/crashtests.list
@@ -59,3 +59,4 @@ load 565248-1.html
 load 558943-1.xhtml
 load 571105-1.xhtml
 load 573127-1.html
+load 580685.html
diff --git a/layout/style/nsRuleNode.cpp b/layout/style/nsRuleNode.cpp
index ee1cc6b457d..1aad6df9eec 100644
--- a/layout/style/nsRuleNode.cpp
+++ b/layout/style/nsRuleNode.cpp
@@ -277,12 +277,15 @@ static nscoord CalcLengthWith(const nsCSSValue& aValue,
         const nsStyleFont *rootStyleFont = styleFont;
         Element* docElement = aPresContext->Document()->GetRootElement();
 
-        rootStyle = aPresContext->StyleSet()->ResolveStyleFor(docElement,
-                                                              nsnull);
-        if (rootStyle) {
-          rootStyleFont = rootStyle->GetStyleFont();
-          rootFontSize = rootStyleFont->mFont.size;
+        if (docElement) {
+          rootStyle = aPresContext->StyleSet()->ResolveStyleFor(docElement,
+                                                                nsnull);
+          if (rootStyle) {
+            rootStyleFont = rootStyle->GetStyleFont();
+          }
         }
+
+        rootFontSize = rootStyleFont->mFont.size;
       }
 
       return ScaleCoord(aValue, float(rootFontSize));
diff --git a/layout/style/test/Makefile.in b/layout/style/test/Makefile.in
index 77607d4a4a0..abc335bbef1 100644
--- a/layout/style/test/Makefile.in
+++ b/layout/style/test/Makefile.in
@@ -132,6 +132,7 @@ _TEST_FILES =	test_acid3_test46.html \
 		test_bug524175.html \
 		test_bug534804.html \
 		test_bug573255.html \
+		test_bug580685.html \
 		test_cascade.html \
 		test_compute_data_with_start_struct.html \
 		test_computed_style_no_pseudo.html \
diff --git a/layout/style/test/test_bug580685.html b/layout/style/test/test_bug580685.html
new file mode 100644
index 00000000000..a887b319490
--- /dev/null
+++ b/layout/style/test/test_bug580685.html
@@ -0,0 +1,42 @@
+<!DOCTYPE HTML>
+<html>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=580685
+-->
+<head>
+  <title>Test for Bug 580685</title>
+  <script type="application/javascript" src="/MochiKit/packed.js"></script>
+  <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
+</head>
+<body>
+<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=580685">Mozilla Bug 580685</a>
+<p id="display">
+  <iframe id="f" src="data:text/html,<body style='outline-offset: 1rem'>">
+  </iframe>
+</p>
+<div id="content" style="display: none">
+  
+</div>
+<pre id="test">
+<script type="application/javascript">
+
+/** Test for Bug 580685 **/
+SimpleTest.waitForExplicitFinish()
+addLoadEvent(function() {
+  var doc = $("f").contentDocument;
+  var b = doc.body;
+  doc.removeChild(doc.documentElement);
+  is(doc.defaultView.getComputedStyle(b, "").outlineOffset,
+     doc.defaultView.getComputedStyle(b, "").fontSize,
+     "1rem did not compute correctly");
+  SimpleTest.finish();
+});
+
+
+
+
+</script>
+</pre>
+</body>
+</html>
