diff --git a/layout/style/crashtests/1247865-1.html b/layout/style/crashtests/1247865-1.html
new file mode 100644
index 000000000000..b7ec8ba60abb
--- /dev/null
+++ b/layout/style/crashtests/1247865-1.html
@@ -0,0 +1,19 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <style>
+    .nav { display: table }
+    .nav:after { content: " " }
+  </style>
+</head>
+<body>
+  <div style="font-size: 1rem"></div>
+  <div class="nav">
+  </div>
+  <script>
+    document.documentElement.style.fontSize = "10px";
+    document.documentElement.offsetHeight;
+    document.documentElement.style.fontSize = "15px";
+  </script>
+</body>
+</html>
diff --git a/layout/style/crashtests/crashtests.list b/layout/style/crashtests/crashtests.list
index 5ae79893972d..2887d7ebe091 100644
--- a/layout/style/crashtests/crashtests.list
+++ b/layout/style/crashtests/crashtests.list
@@ -136,6 +136,7 @@ load 1230408-1.html
 load 1233135-1.html
 load 1233135-2.html
 load 1238660-1.html
+asserts(1) load 1247865-1.html # bug 1247865
 load border-image-visited-link.html
 load font-face-truncated-src.html 
 load large_border_image_width.html
