new file mode 100644
--- /dev/null
+++ b/layout/reftests/reftest-sanity/default-preferences-include.list
@@ -0,0 +1,6 @@
+# test default-preferences on include commands
+
+# In default-preferences-tests.list, the default-preferences line in effect
+# before the include statement sets different font sizes for the test and
+# reference files. Those default preferences should not inherit into this file.
+== font-default.html font-default.html

new file mode 100644
--- /dev/null
+++ b/layout/reftests/reftest-sanity/default-preferences-tests.list
@@ -0,0 +1,28 @@
+# test default-preferences
+
+# test default-preferences with a pref()
+default-preferences pref(font.size.variable.x-western,24)
+!= font-default.html font-size-16.html
+== font-default.html font-size-24.html
+
+# test that a default preference can be overridden
+pref(font.size.variable.x-western,16) == font-default.html font-size-16.html
+pref(font.size.variable.x-western,16) != font-default.html font-size-24.html
+
+# test that default preferences are kept when other test-specific preferences are set
+pref(font.size.variable.zh-HK,36) != font-default.html font-size-16.html
+pref(font.size.variable.zh-HK,36) == font-default.html font-size-24.html
+
+# test default-preferences with test-pref() and ref-pref()
+default-preferences test-pref(font.size.variable.x-western,16) ref-pref(font.size.variable.x-western,24)
+!= font-default.html font-default.html
+== font-default.html font-size-16.html
+== font-size-24.html font-default.html
+
+# test that default-preferences does not apply to include commands
+include default-preferences-include.list
+
+# test resetting default-preferences
+default-preferences
+== font-default.html font-default.html
+

--- a/layout/reftests/reftest-sanity/reftest.list
+++ b/layout/reftests/reftest-sanity/reftest.list
@@ -60,16 +60,19 @@ asserts-if(false,6-8) load about:blank
 == div.html div-ref.html
 
 # test script keyword
 include scripttests.list
 
 # test url-prefix
 include urlprefixtests.list
 
+# test default-preferences
+include default-preferences-tests.list
+
 # test that all corners are visible
 != corners-1.html corners-1-ref.html
 != corners-2.html corners-2-ref.html
 != corners-3.html corners-3-ref.html
 != corners-4.html corners-4-ref.html
 
 # Test that the harness gives the correct page dimensions.
 != page-width-3.9in.html page-width-4in.html

--- a/layout/tools/reftest/README.txt
+++ b/layout/tools/reftest/README.txt
@@ -285,16 +285,38 @@ 3. Specification of a url prefix
    <string> will not be prepended to any <url> or <url_ref> matching the pattern
    /^\w+:/. This will prevent the prefix from being applied to any absolute url
    containing a protocol such as data:, about:, or http:.
 
    While the typical use of url-prefix is expected to be as the first line of
    a manifest, it is legal to use it anywhere in a manifest. Subsequent uses
    of url-prefix overwrite any existing values.
 
+4. Specification of default preferences
+
+   default-preferences <preference>*
+
+   where <preference> is defined above.
+
+   The <preference> settings will be used for all following test items in the
+   manifest.
+
+   If a test item includes its own preference settings, then they will override
+   any settings for preferences of the same names that are set using
+   default-preferences, just as later items within a line override earlier ones.
+
+   A default-preferences line with no <preference> settings following it will
+   reset the set of default preferences to be empty.
+
+   As with url-prefix, default-preferences will often be used at the start of a
+   manifest file so that it applies to all test items, but it is legal for
+   default-preferences to appear anywhere in the manifest. A subsequent
+   default-preferences will reset any previous default preference values and
+   overwrite them with the specified <preference> values.
+
 This test manifest format could be used by other harnesses, such as ones
 that do not depend on XUL, or even ones testing other layout engines.
 
 Running Tests
 =============
 
 (If you're not using a DEBUG build, first set browser.dom.window.dump.enabled
 to true (in about:config, in the profile you'll be using to run the tests).

--- a/layout/tools/reftest/reftest.js
+++ b/layout/tools/reftest/reftest.js
@@ -125,16 +125,17 @@ const EXPECTED_FUZZY = 4;
 // types of preference value we might want to set for a specific test
 const PREF_BOOLEAN = 0;
 const PREF_STRING  = 1;
 const PREF_INTEGER = 2;
 
 var gPrefsToRestore = [];
 
 const gProtocolRE = /^\w+:/;
+const gPrefItemRE = /^(|test-|ref-)pref\((.+?),(.*)\)$/;
 
 var HTTP_SERVER_PORT = 4444;
 const HTTP_SERVER_PORTS_TO_TRY = 50;
 
 // whether to run slow tests or not
 var gRunSlowTests = true;
 
 // whether we should skip caching canvases
@@ -649,16 +650,42 @@ function BuildConditionSandbox(aURL) {
     if (!gDumpedConditionSandbox) {
         dump("REFTEST INFO | Dumping JSON representation of sandbox \n");
         dump("REFTEST INFO | " + JSON.stringify(sandbox) + " \n");
         gDumpedConditionSandbox = true;
     }
     return sandbox;
 }
 
+function AddPrefSettings(aWhere, aPrefName, aPrefValExpression, aSandbox, aTestPrefSettings, aRefPrefSettings)
+{
+    var prefVal = Components.utils.evalInSandbox("(" + aPrefValExpression + ")", aSandbox);
+    var prefType;
+    var valType = typeof(prefVal);
+    if (valType == "boolean") {
+        prefType = PREF_BOOLEAN;
+    } else if (valType == "string") {
+        prefType = PREF_STRING;
+    } else if (valType == "number" && (parseInt(prefVal) == prefVal)) {
+        prefType = PREF_INTEGER;
+    } else {
+        return false;
+    }
+    var setting = { name: aPrefName,
+                    type: prefType,
+                    value: prefVal };
+    if (aWhere != "ref-") {
+        aTestPrefSettings.push(setting);
+    }
+    if (aWhere != "test-") {
+        aRefPrefSettings.push(setting);
+    }
+    return true;
+}
+
 function ReadTopManifest(aFileURL)
 {
     gURLs = new Array();
     var url = gIOService.newURI(aFileURL, null, null);
     if (!url)
         throw "Expected a file or http URL for the manifest.";
     ReadManifest(url, EXPECTED_PASS);
 }
@@ -688,16 +715,17 @@ function ReadManifest(aURL, inherited_st
     var streamBuf = getStreamContent(inputStream);
     inputStream.close();
     var lines = streamBuf.split(/\n|\r|\r\n/);
 
     // Build the sandbox for fails-if(), etc., condition evaluation.
     var sandbox = BuildConditionSandbox(aURL);
     var lineNo = 0;
     var urlprefix = "";
+    var defaultTestPrefSettings = [], defaultRefPrefSettings = [];
     for each (var str in lines) {
         ++lineNo;
         if (str.charAt(0) == "#")
             continue; // entire line was a comment
         var i = str.search(/\s+#/);
         if (i >= 0)
             str = str.substring(0, i);
         // strip leading and trailing whitespace
@@ -708,23 +736,41 @@ function ReadManifest(aURL, inherited_st
 
         if (items[0] == "url-prefix") {
             if (items.length != 2)
                 throw "url-prefix requires one url in manifest file " + aURL.spec + " line " + lineNo;
             urlprefix = items[1];
             continue;
         }
 
+        if (items[0] == "default-preferences") {
+            var m;
+            var item;
+            defaultTestPrefSettings = [];
+            defaultRefPrefSettings = [];
+            items.shift();
+            while ((item = items.shift())) {
+                if (!(m = item.match(gPrefItemRE))) {
+                    throw "Unexpected item in default-preferences list in manifest file " + aURL.spec + " line " + lineNo;
+                }
+                if (!AddPrefSettings(m[1], m[2], m[3], sandbox, defaultTestPrefSettings, defaultRefPrefSettings)) {
+                    throw "Error in pref value in manifest file " + aURL.spec + " line " + lineNo;
+                }
+            }
+            continue;
+        }
+
         var expected_status = EXPECTED_PASS;
         var allow_silent_fail = false;
         var minAsserts = 0;
         var maxAsserts = 0;
         var needs_focus = false;
         var slow = false;
-        var testPrefSettings = [], refPrefSettings = [];
+        var testPrefSettings = defaultTestPrefSettings.concat();
+        var refPrefSettings = defaultRefPrefSettings.concat();
         var fuzzy_max_delta = 2;
         var fuzzy_max_pixels = 1;
 
         while (items[0].match(/^(fails|needs-focus|random|skip|asserts|slow|require-or|silentfail|pref|test-pref|ref-pref|fuzzy)/)) {
             var item = items.shift();
             var stat;
             var cond;
             var m = item.match(/^(fails|random|skip|silentfail)-if(\(.*\))$/);
@@ -780,41 +826,21 @@ function ReadManifest(aURL, inherited_st
                 }
             } else if ((m = item.match(/^slow-if\((.*?)\)$/))) {
                 cond = false;
                 if (Components.utils.evalInSandbox("(" + m[1] + ")", sandbox))
                     slow = true;
             } else if (item == "silentfail") {
                 cond = false;
                 allow_silent_fail = true;
-            } else if ((m = item.match(/^(|test-|ref-)pref\((.+?),(.*)\)$/))) {
+            } else if ((m = item.match(gPrefItemRE))) {
                 cond = false;
-                var where = m[1];
-                var prefName = m[2];
-                var prefVal = Components.utils.evalInSandbox("(" + m[3] + ")", sandbox);
-                var prefType;
-                var valType = typeof(prefVal);
-                if (valType == "boolean") {
-                    prefType = PREF_BOOLEAN;
-                } else if (valType == "string") {
-                    prefType = PREF_STRING;
-                } else if (valType == "number" && (parseInt(prefVal) == prefVal)) {
-                    prefType = PREF_INTEGER;
-                } else {
+                if (!AddPrefSettings(m[1], m[2], m[3], sandbox, testPrefSettings, refPrefSettings)) {
                     throw "Error in pref value in manifest file " + aURL.spec + " line " + lineNo;
                 }
-                var setting = { name: prefName,
-                                type: prefType,
-                                value: prefVal };
-                if (where != "ref-") {
-                    testPrefSettings.push(setting);
-                }
-                if (where != "test-") {
-                    refPrefSettings.push(setting);
-                }
             } else if ((m = item.match(/^fuzzy\((\d+),(\d+)\)$/))) {
               cond = false;
               expected_status = EXPECTED_FUZZY;
               fuzzy_max_delta = Number(m[1]);
               fuzzy_max_pixels = Number(m[2]);
             } else if ((m = item.match(/^fuzzy-if\((.*?),(\d+),(\d+)\)$/))) {
               cond = false;
               if (Components.utils.evalInSandbox("(" + m[1] + ")", sandbox)) {