--- a/layout/style/test/test_transitions_per_property.html
+++ b/layout/style/test/test_transitions_per_property.html
@@ -82,39 +82,27 @@ var supported_properties = {
     "-moz-image-region": [ test_rect_transition ],
     "-moz-outline-radius-bottomleft": [ test_radius_transition ],
     "-moz-outline-radius-bottomright": [ test_radius_transition ],
     "-moz-outline-radius-topleft": [ test_radius_transition ],
     "-moz-outline-radius-topright": [ test_radius_transition ],
     "background-color": [ test_color_transition,
                           test_true_currentcolor_transition ],
     "background-position": [ test_background_position_transition,
-                             // FIXME: We don't currently test clamping,
-                             // since background-position uses calc() as
-                             // an intermediate form.
-                             /* test_length_percent_pair_unclamped */ ],
+                             test_length_percent_pair_unclamped ],
     "background-position-x": [ test_background_position_coord_transition,
                                test_length_transition,
                                test_percent_transition,
-                             // FIXME: We don't currently test clamping,
-                             // since background-position-x uses calc() as
-                             // an intermediate form.
-                             /* test_length_percent_pair_unclamped */ ],
+                               test_length_percent_pair_unclamped ],
     "background-position-y": [ test_background_position_coord_transition,
                                test_length_transition,
                                test_percent_transition,
-                             // FIXME: We don't currently test clamping,
-                             // since background-position-y uses calc() as
-                             // an intermediate form.
-                             /* test_length_percent_pair_unclamped */ ],
+                               test_length_percent_pair_unclamped ],
     "background-size": [ test_background_size_transition,
-                         // FIXME: We don't currently test clamping,
-                         // since background-size uses calc() as an
-                         // intermediate form.
-                         /* test_length_percent_pair_clamped */ ],
+                         test_length_percent_pair_clamped ],
     "border-bottom-color": [ test_color_transition,
                              test_true_currentcolor_transition ],
     "border-bottom-width": [ test_length_transition,
                              test_length_clamped ],
     "border-left-color": [ test_color_transition,
                            test_true_currentcolor_transition ],
     "border-left-width": [ test_length_transition,
                            test_length_clamped ],
@@ -190,39 +178,27 @@ var supported_properties = {
                      test_length_unclamped, test_percent_unclamped ],
     "margin-right": [ test_length_transition, test_percent_transition,
                       test_length_percent_calc_transition,
                       test_length_unclamped, test_percent_unclamped ],
     "margin-top": [ test_length_transition, test_percent_transition,
                     test_length_percent_calc_transition,
                     test_length_unclamped, test_percent_unclamped ],
     "mask-position": [ test_background_position_transition,
-                       // FIXME: We don't currently test clamping,
-                       // since mask-position uses calc() as
-                       // an intermediate form.
-                       /* test_length_percent_pair_unclamped */ ],
+                       test_length_percent_pair_unclamped ],
     "mask-position-x": [ test_background_position_coord_transition,
                          test_length_transition,
                          test_percent_transition,
-                         // FIXME: We don't currently test clamping,
-                         // since background-position-x uses calc() as
-                         // an intermediate form.
-                         /* test_length_percent_pair_unclamped */ ],
+                         test_length_percent_pair_unclamped ],
     "mask-position-y": [ test_background_position_coord_transition,
                          test_length_transition,
                          test_percent_transition,
-                         // FIXME: We don't currently test clamping,
-                         // since background-position-y uses calc() as
-                         // an intermediate form.
-                         /* test_length_percent_pair_unclamped */ ],
+                         test_length_percent_pair_unclamped ],
     "mask-size": [ test_background_size_transition,
-                   // FIXME: We don't currently test clamping,
-                   // since mask-size uses calc() as an
-                   // intermediate form.
-                   /* test_length_percent_pair_clamped */ ],
+                   test_length_percent_pair_clamped ],
     "max-height": [ test_length_transition, test_percent_transition,
                     test_length_clamped, test_percent_clamped ],
     "max-width": [ test_length_transition, test_percent_transition,
                    test_length_clamped, test_percent_clamped ],
     "min-height": [ test_length_transition, test_percent_transition,
                     test_length_clamped, test_percent_clamped ],
     "min-width": [ test_length_transition, test_percent_transition,
                    test_length_clamped, test_percent_clamped ],
@@ -2114,21 +2090,28 @@ function test_length_percent_pair_clampe
 function test_length_percent_pair_unclamped(prop) {
   test_length_percent_pair_clamped_or_unclamped(prop, false);
 }
 
 function test_length_percent_pair_clamped_or_unclamped(prop, is_clamped) {
   div.style.setProperty("transition-timing-function", FUNC_NEGATIVE, "");
   div.style.setProperty("transition-property", "none", "");
   div.style.setProperty(prop, "0px 0%", "");
-  is(cs.getPropertyValue(prop), "0px 0px",
+  var is_zero = function(val) {
+    if (prop == "transform-origin" || prop == "perspective-origin") {
+      // These two properties resolve percentages to pixels.
+      return val == "0px 0px";
+    }
+    return val == "0px 0%";
+  }
+  ok(is_zero(cs.getPropertyValue(prop)),
      "length+percent-valued property " + prop + ": flush before clamping test");
   div.style.setProperty("transition-property", prop, "");
   div.style.setProperty(prop, "30px 25%", "");
-  (is_clamped ? is : isnot)(cs.getPropertyValue(prop), "0px 0px",
+  is(is_zero(cs.getPropertyValue(prop)), is_clamped,
      "length+percent-valued property " + prop + ": clamping of negatives");
   div.style.setProperty("transition-timing-function", "linear", "");
 }
 
 function test_rect_transition(prop) {
   div.style.setProperty("transition-property", "none", "");
   div.style.setProperty(prop, "rect(4px, 16px, 12px, 6px)", "");
   is(cs.getPropertyValue(prop), "rect(4px, 16px, 12px, 6px)",