--- a/dom/media/tests/mochitest/test_getUserMedia_constraints.html
+++ b/dom/media/tests/mochitest/test_getUserMedia_constraints.html
@@ -65,25 +65,27 @@ var tests = [
     constraints: { video: { mandatory: { facingMode: 'left' } } },
     error: null },
 ];
 
 var mustSupport = [
   'width', 'height', 'frameRate', 'facingMode', 'deviceId',
   // Yet to add:
   //  'aspectRatio', 'frameRate', 'volume', 'sampleRate', 'sampleSize',
-  //  'echoCancellation', 'latency', 'groupId'
+  //  'latency', 'groupId'
 
   // http://fluffy.github.io/w3c-screen-share/#screen-based-video-constraints
   // OBE by http://w3c.github.io/mediacapture-screen-share
   'mediaSource',
 
   // Experimental https://bugzilla.mozilla.org/show_bug.cgi?id=1131568#c3
   'browserWindow', 'scrollWithPage',
-  'viewportOffsetX', 'viewportOffsetY', 'viewportWidth', 'viewportHeight'
+  'viewportOffsetX', 'viewportOffsetY', 'viewportWidth', 'viewportHeight',
+
+  'echoCancellation'
 ];
 
 var mustFailWith = (msg, reason, constraint, f) =>
   f().then(() => ok(false, msg + " must fail"), e => {
     is(e.name, reason, msg + " must fail: " + e.message);
     if (constraint !== undefined) {
       is(e.constraint, constraint, msg + " must fail w/correct constraint.");
     }

--- a/dom/webidl/MediaStreamTrack.webidl
+++ b/dom/webidl/MediaStreamTrack.webidl
@@ -49,16 +49,17 @@ dictionary MediaTrackConstraintSet {
     DOMString mediaSource = "camera";
     long long browserWindow;
     boolean scrollWithPage;
     ConstrainDOMString deviceId;
     ConstrainLong viewportOffsetX;
     ConstrainLong viewportOffsetY;
     ConstrainLong viewportWidth;
     ConstrainLong viewportHeight;
+    ConstrainBoolean echoCancellation;
 };
 
 dictionary MediaTrackConstraints : MediaTrackConstraintSet {
     sequence<MediaTrackConstraintSet> advanced;
 };
 
 [Exposed=Window]
 interface MediaStreamTrack : EventTarget {

--- a/dom/webidl/MediaTrackSupportedConstraints.webidl
+++ b/dom/webidl/MediaTrackSupportedConstraints.webidl
@@ -11,17 +11,17 @@ dictionary MediaTrackSupportedConstraint
     boolean width = true;
     boolean height = true;
     boolean aspectRatio;        // to be supported
     boolean frameRate = true;
     boolean facingMode = true;
     boolean volume;             // to be supported
     boolean sampleRate;         // to be supported
     boolean sampleSize;         // to be supported
-    boolean echoCancellation;   // to be supported
+    boolean echoCancellation = true;
     boolean latency;            // to be supported
     boolean deviceId = true;
     boolean groupId;            // to be supported
 
     // Mozilla-specific extensions:
 
     // http://fluffy.github.io/w3c-screen-share/#screen-based-video-constraints
     // OBE by http://w3c.github.io/mediacapture-screen-share