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+<!DOCTYPE HTML>
+<html>
+<head>
+  <title>Test for correct handling of aStartStruct parameter to nsRuleNode::Compute*Data</title>
+  <script type="text/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <script type="text/javascript" src="property_database.js"></script>
+  <style type="text/css" id="stylesheet"></style>
+  <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=216456">Mozilla Bug 216456</a>
+<p id="display">
+  <span id="base"></span>
+  <span id="test"></span>
+</p>
+<div id="content" style="display: none">
+  
+</div>
+<pre id="test">
+<script class="testbody" type="text/javascript">
+
+/**
+ * The purpose of this test is to test that nsRuleNode::Compute*Data
+ * functions are written correctly.  In particular, in these functions,
+ * when the specified value of a property has unit eCSSUnit_Null,
+ * touching the computed data is forbidden.  This is because we
+ * sometimes stop walking up the rule tree when we find computed data
+ * for an initial subsequence of our rules (i.e., an ancestor rule node)
+ * that we can use as a starting point (aStartStruct) for the
+ * computation for the current rule node.
+ *
+ * If one of these tests fails, you should look for a case where the
+ * property's code in nsRuleNode::Compute*Data touches the computed
+ * value when the specified value has eCSSUnit_Null, and fix it.
+ *
+ * The test works by maintaining one style rule that has every CSS
+ * property specified, and a second style rule that has different values
+ * for every property, an element that matches only the first rule (so
+ * that we'll have a cached struct), and *later* an element that matches
+ * both rules (for whose computation we'll find the struct cached from
+ * the first element).  (It does this twice, once with the overriding in
+ * each direction, because one of the cases might match the incorrect
+ * overwriting.)  Then, in the second style rule, it unsets each
+ * property, one at a time, and checks that the computation works
+ * correctly.  (The reason to want every property set is to hit a case
+ * where we can store the data in the rule tree... though this isn't
+ * guaranteed.)
+ */
+
+function xfail_computecheck(prop, roundnum) {
+    return false;
+}
+
+function xfail_test(prop, roundnum) {
+    return false;
+}
+
+var gStyleSheet = document.getElementById("stylesheet").sheet;
+var gRule1 = gStyleSheet.cssRules[gStyleSheet.insertRule("#base, #test {}", gStyleSheet.cssRules.length)];
+var gRule2 = gStyleSheet.cssRules[gStyleSheet.insertRule("#test {}", gStyleSheet.cssRules.length)];
+
+var gBase = getComputedStyle(document.getElementById("base"), "");
+var gTest = getComputedStyle(document.getElementById("test"), "");
+
+function round(lower_set, higher_set, roundnum) {
+
+  for (var prop in gCSSProperties) {
+    var info = gCSSProperties[prop];
+    if (info.subproperties || info.get_computed)
+      continue;
+    gRule1.style.setProperty(prop, info[lower_set][0], "");
+    gRule2.style.setProperty(prop, info[higher_set][0], "");
+  }
+
+  for (var prop in gCSSProperties) {
+    var info = gCSSProperties[prop];
+    if (info.subproperties || info.get_computed)
+      continue;
+
+    if ("prerequisites" in info) {
+      for (var prereq in info.prerequisites) {
+        gRule2.style.setProperty(prereq, info.prerequisites[prereq], "");
+      }
+    }
+
+    gBase.getPropertyValue(prop);
+    var higher_set_val = gTest.getPropertyValue(prop);
+    gRule2.style.setProperty(prop, info[lower_set][0], "");
+    var lower_set_val = gTest.getPropertyValue(prop);
+    (xfail_computecheck(prop, roundnum) ? todo_isnot : isnot)(higher_set_val, lower_set_val, "initial and other values of " + prop + " are different");
+    gRule2.style.removeProperty(prop);
+    (xfail_test(prop, roundnum) ? todo_is : is)(gTest.getPropertyValue(prop), lower_set_val, prop + " is not touched when its value comes from aStartStruct");
+
+    gRule2.style.setProperty(prop, info[higher_set][0], "");
+    if ("prerequisites" in info) {
+      for (var prereq in info.prerequisites) {
+        gRule2.style.setProperty(prereq, gCSSProperties[prereq][higher_set][0], "");
+      }
+    }
+  }
+}
+
+round("other_values", "initial_values", 1);
+round("initial_values", "other_values", 2);
+
+</script>
+</pre>
+</body>
+</html>