--- a/dom/media/test/test_texttrackcue.html
+++ b/dom/media/test/test_texttrackcue.html
@@ -84,34 +84,37 @@ SpecialPowers.pushPrefEnv({"set": [["med
       cue.pauseOnExit = true;
       is(cue.pauseOnExit, true, "Cue's pause on exit flag should be true.");
       video.addEventListener("pause", function pauseOnExit() {
         video.removeEventListener("pause", pauseOnExit, false);
         video.play();
       });
 
       var exceptionHappened;
-      function checkPercentageValue(prop) {
+      function checkPercentageValue(prop, initialVal) {
         ok(prop in cue, prop + " should be a property on VTTCue.");
-        cue[prop] = 20;
-        is(cue[prop], 20, "Cue's " + prop + " should now be 20.");
+        cue[prop] = initialVal;
+        is(cue[prop], initialVal, "Cue's " + prop + " should now be " + initialVal);
         [ 101, -1 ].forEach(function(val) {
           exceptionHappened = false;
           try {
             cue[prop] = val;
           } catch(e) {
             exceptionHappened = true;
             is(e.name, "IndexSizeError", "Should have thrown IndexSizeError.");
           }
           ok(exceptionHappened, "Exception should have happened.");
         });
       }
 
-      checkPercentageValue("size");
-      checkPercentageValue("position");
+      checkPercentageValue("size", 100.0);
+      cue.size = 50.5;
+      is(cue.size, 50.5, "Cue's size should be 50.5.")
+
+      checkPercentageValue("position", 50.0);
 
       ok(cue.snapToLines, "Cue's snapToLines should be set by set.");
       cue.snapToLines = false;
       ok(!cue.snapToLines, "Cue's snapToLines should not be set.");
 
       function checkEnumValue(prop, initialVal, acceptedValues) {
         ok(prop in cue, prop + " should be a property on VTTCue.");
         is(cue[prop], initialVal, "Cue's " + prop + " should be " + initialVal);