--- a/layout/style/test/test_transitions_per_property.html
+++ b/layout/style/test/test_transitions_per_property.html
@@ -58,24 +58,16 @@ function any_unit_to_num(str)
 var FUNC_NEGATIVE = "cubic-bezier(0.25, -2, 0.75, 1)";
 var FUNC_OVERONE = "cubic-bezier(0.25, 0, 0.75, 3)";
 
 // After resolving Bug 1387080, we can remove this variable.
 const isServo = SpecialPowers.DOMWindowUtils.isStyledByServo;
 
 // The following properties will be skipped while using Servo backend.
 var skippedProperties = {
-    "background-position": true,    // Bug 1387946
-    "background-position-x": true,  // Bug 1387946
-    "background-position-y": true,  // Bug 1387946
-    "background-size": true,        // Bug 1387946
-    "mask-position": true,          // Bug 1387946
-    "mask-position-x": true,        // Bug 1387946
-    "mask-position-y": true,        // Bug 1387946
-    "mask-size": true,              // Bug 1387946
     "box-shadow": true,             // Bug 1387973
     "stroke-dasharray": true,       // Bug 1387986
     "stroke-dashoffset": true,      // Bug 1369614
     "stroke-width": true,           // Bug 1369614
     "text-shadow": true,            // Bug 1387973
 };
 
 var supported_properties = {
@@ -2333,46 +2325,57 @@ function test_background_position_coord_
   div.style.setProperty("transition-property", prop, "");
   div.style.setProperty(prop, "calc(40px + 20%)", "");
   is(cs.getPropertyValue(prop), "calc(-5px + 80%)",
      "property " + prop + ": interpolation of edge keywords w/ offsets & calc");
   check_distance(prop, `${endEdge} 20px`,
                        "calc(-5px + 80%)",
                        "calc(40px + 20%)");
 
+  div.style.setProperty("transition-property", "none", "");
   div.style.setProperty(prop, "10px, 50px, 30px", "");
   is(cs.getPropertyValue(prop), "10px, 50px, 30px",
      "property " + prop + ": computed value before transition");
+  div.style.setProperty("transition-property", prop, "");
   div.style.setProperty(prop, "50px, 70px, 30px", "");
   is(cs.getPropertyValue(prop), "20px, 55px, 30px",
      "property " + prop + ": interpolation of lists of lengths");
   check_distance(prop, "10px, 50px, 30px",
                        "20px, 55px, 30px",
                        "50px, 70px, 30px");
+
+  div.style.setProperty("transition-property", "none", "");
   div.style.setProperty(prop, "10px, 50%, 30%, 5px", "");
   is(cs.getPropertyValue(prop), "10px, 50%, 30%, 5px",
      "property " + prop + ": computed value before transition");
+  div.style.setProperty("transition-property", prop, "");
   div.style.setProperty(prop, "50px, 70%, 30%, 25px", "");
   is(cs.getPropertyValue(prop), "20px, 55%, 30%, 10px",
      "property " + prop + ": interpolation of lists of lengths and percents");
   check_distance(prop, "10px, 50%, 30%, 5px",
                        "20px, 55%, 30%, 10px",
                        "50px, 70%, 30%, 25px");
+
+  div.style.setProperty("transition-property", "none", "");
   div.style.setProperty(prop, "20%, 8px", "");
   is(cs.getPropertyValue(prop), "20%, 8px",
      "property " + prop + ": computed value before transition");
+  div.style.setProperty("transition-property", prop, "");
   div.style.setProperty(prop, "12px, 40%", "");
   is(cs.getPropertyValue(prop), "calc(3px + 15%), calc(6px + 10%)",
      "property " + prop + ": interpolation that computes to calc()");
   check_distance(prop, "20%, 8px",
                        "calc(3px + 15%), calc(6px + 10%)",
                        "12px, 40%");
+
+  div.style.setProperty("transition-property", "none", "");
   div.style.setProperty(prop, "calc(20% + 40px), 8px, calc(20px + 12%)", "");
   is(cs.getPropertyValue(prop), "calc(40px + 20%), 8px, calc(20px + 12%)",
      "property " + prop + ": computed value before transition");
+  div.style.setProperty("transition-property", prop, "");
   div.style.setProperty(prop, "12px, calc(20%), calc(8px + 20%)", "");
   is(cs.getPropertyValue(prop), "calc(33px + 15%), calc(6px + 5%), calc(17px + 14%)",
      "property " + prop + ": interpolation that computes to calc()");
   check_distance(prop, "calc(20% + 40px), 8px, calc(20px + 12%)",
                        "calc(33px + 15%), calc(6px + 5%), calc(17px + 14%)",
                        "12px, calc(20%), calc(8px + 20%)");
 }
 
@@ -2467,52 +2470,60 @@ function test_background_position_size_c
   if (doesPropHaveDistanceComputation) {
     check_distance(prop, "calc(40px + 10%) 16px",
                          "calc(30px + 15%) calc(14px + 15%)",
                          "30% calc(8px + 60%)");
   }
 
   // Test list values, if appropriate
   if (doesPropTakeListValues) {
+    div.style.setProperty("transition-property", "none", "");
     div.style.setProperty(prop, "10px 40px, 50px 50px, 30px 20px", "");
     is(cs.getPropertyValue(prop), "10px 40px, 50px 50px, 30px 20px",
        "property " + prop + ": computed value before transition");
+    div.style.setProperty("transition-property", prop, "");
     div.style.setProperty(prop, "50px 20px, 70px 50px, 30px 40px", "");
     is(cs.getPropertyValue(prop), "20px 35px, 55px 50px, 30px 25px",
        "property " + prop + ": interpolation of lists of lengths");
     if (doesPropHaveDistanceComputation) {
       check_distance(prop, "10px 40px, 50px 50px, 30px 20px",
                            "20px 35px, 55px 50px, 30px 25px",
                            "50px 20px, 70px 50px, 30px 40px");
     }
+    div.style.setProperty("transition-property", "none", "");
     div.style.setProperty(prop, "10px 40%, 50% 50px, 30% 20%, 5px 10px", "");
     is(cs.getPropertyValue(prop), "10px 40%, 50% 50px, 30% 20%, 5px 10px",
        "property " + prop + ": computed value before transition");
+    div.style.setProperty("transition-property", prop, "");
     div.style.setProperty(prop, "50px 20%, 70% 50px, 30% 40%, 25px 50px", "");
     is(cs.getPropertyValue(prop), "20px 35%, 55% 50px, 30% 25%, 10px 20px",
        "property " + prop + ": interpolation of lists of lengths and percents");
     if (doesPropHaveDistanceComputation) {
       check_distance(prop, "10px 40%, 50% 50px, 30% 20%, 5px 10px",
                            "20px 35%, 55% 50px, 30% 25%, 10px 20px",
                            "50px 20%, 70% 50px, 30% 40%, 25px 50px");
     }
+    div.style.setProperty("transition-property", "none", "");
     div.style.setProperty(prop, "20% 40%, 8px 12px", "");
     is(cs.getPropertyValue(prop), "20% 40%, 8px 12px",
        "property " + prop + ": computed value before transition");
+    div.style.setProperty("transition-property", prop, "");
     div.style.setProperty(prop, "12px 20px, 40% 16%", "");
     is(cs.getPropertyValue(prop), "calc(3px + 15%) calc(5px + 30%), calc(6px + 10%) calc(9px + 4%)",
        "property " + prop + ": interpolation that computes to calc()");
     if (doesPropHaveDistanceComputation) {
       check_distance(prop, "20% 40%, 8px 12px",
                            "calc(3px + 15%) calc(5px + 30%), calc(6px + 10%) calc(9px + 4%)",
                            "12px 20px, 40% 16%");
     }
+    div.style.setProperty("transition-property", "none", "");
     div.style.setProperty(prop, "calc(20% + 40px) calc(40px + 40%), 8px 12%, calc(20px + 12%) calc(24px + 8%)", "");
     is(cs.getPropertyValue(prop), "calc(40px + 20%) calc(40px + 40%), 8px 12%, calc(20px + 12%) calc(24px + 8%)",
        "property " + prop + ": computed value before transition");
+    div.style.setProperty("transition-property", prop, "");
     div.style.setProperty(prop, "12px 20%, calc(20%) calc(16px + 60%), calc(8px + 20%) calc(40px + 16%)", "");
     is(cs.getPropertyValue(prop), "calc(33px + 15%) calc(30px + 35%), calc(6px + 5%) calc(4px + 24%), calc(17px + 14%) calc(28px + 10%)",
        "property " + prop + ": interpolation that computes to calc()");
     if (doesPropHaveDistanceComputation) {
       check_distance(prop, "calc(20% + 40px) calc(40px + 40%), 8px 12%, calc(20px + 12%) calc(24px + 8%)",
                            "calc(33px + 15%) calc(30px + 35%), calc(6px + 5%) calc(4px + 24%), calc(17px + 14%) calc(28px + 10%)",
                            "12px 20%, calc(20%) calc(16px + 60%), calc(8px + 20%) calc(40px + 16%)");
     }