--- a/testing/marionette/harness/marionette_harness/tests/unit/test_capabilities.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_capabilities.py
@@ -62,24 +62,16 @@ class TestCapabilities(MarionetteTestCas
                 current_profile = self.marionette.instance.runner.device.app_ctx.remote_profile
             else:
                 current_profile = convert_path(self.marionette.instance.runner.profile.profile)
             self.assertEqual(convert_path(str(self.caps["moz:profile"])), current_profile)
             self.assertEqual(convert_path(str(self.marionette.profile)), current_profile)
 
         self.assertIn("moz:accessibilityChecks", self.caps)
         self.assertFalse(self.caps["moz:accessibilityChecks"])
-        self.assertIn("specificationLevel", self.caps)
-        self.assertEqual(self.caps["specificationLevel"], 0)
-
-    def test_set_specification_level(self):
-        self.marionette.delete_session()
-        self.marionette.start_session({"specificationLevel": 2})
-        caps = self.marionette.session_capabilities
-        self.assertEqual(2, caps["specificationLevel"])
 
     def test_we_get_valid_uuid4_when_creating_a_session(self):
         self.assertNotIn("{", self.marionette.session_id,
                          "Session ID has {{}} in it: {}".format(
                              self.marionette.session_id))
 
 
 class TestCapabilityMatching(MarionetteTestCase):

--- a/testing/marionette/session.js
+++ b/testing/marionette/session.js
@@ -262,17 +262,16 @@ session.Capabilities = class extends Map
       ["acceptInsecureCerts", false],
       ["timeouts", new session.Timeouts()],
       ["proxy", new session.Proxy()],
 
       // features
       ["rotatable", appinfo.name == "B2G"],
 
       // proprietary
-      ["specificationLevel", 0],
       ["moz:processID", Services.appinfo.processID],
       ["moz:profile", maybeProfile()],
       ["moz:accessibilityChecks", false],
       ["moz:headless", Cc["@mozilla.org/gfx/info;1"].getService(Ci.nsIGfxInfo).isHeadless],
     ]);
   }
 
   /**
@@ -353,21 +352,16 @@ session.Capabilities = class extends Map
           matched.set("proxy", proxy);
           break;
 
         case "timeouts":
           let timeouts = session.Timeouts.fromJSON(v);
           matched.set("timeouts", timeouts);
           break;
 
-        case "specificationLevel":
-          assert.positiveInteger(v);
-          matched.set("specificationLevel", v);
-          break;
-
         case "moz:accessibilityChecks":
           assert.boolean(v);
           matched.set("moz:accessibilityChecks", v);
           break;
       }
     }
 
     return matched;

--- a/testing/marionette/test_session.js
+++ b/testing/marionette/test_session.js
@@ -236,17 +236,16 @@ add_test(function test_Capabilities_ctor
   ok(caps.has("platformVersion"));
   equal(session.PageLoadStrategy.Normal, caps.get("pageLoadStrategy"));
   equal(false, caps.get("acceptInsecureCerts"));
   ok(caps.get("timeouts") instanceof session.Timeouts);
   ok(caps.get("proxy") instanceof session.Proxy);
 
   ok(caps.has("rotatable"));
 
-  equal(0, caps.get("specificationLevel"));
   ok(caps.has("moz:processID"));
   ok(caps.has("moz:profile"));
   equal(false, caps.get("moz:accessibilityChecks"));
 
   run_next_test();
 });
 
 add_test(function test_Capabilities_toString() {
@@ -265,17 +264,16 @@ add_test(function test_Capabilities_toJS
   equal(caps.get("platformVersion"), json.platformVersion);
   equal(caps.get("pageLoadStrategy"), json.pageLoadStrategy);
   equal(caps.get("acceptInsecureCerts"), json.acceptInsecureCerts);
   deepEqual(caps.get("timeouts").toJSON(), json.timeouts);
   equal(undefined, json.proxy);
 
   equal(caps.get("rotatable"), json.rotatable);
 
-  equal(caps.get("specificationLevel"), json.specificationLevel);
   equal(caps.get("moz:processID"), json["moz:processID"]);
   equal(caps.get("moz:profile"), json["moz:profile"]);
   equal(caps.get("moz:accessibilityChecks"), json["moz:accessibilityChecks"]);
 
   run_next_test();
 });
 
 add_test(function test_Capabilities_fromJSON() {
@@ -307,22 +305,16 @@ add_test(function test_Capabilities_from
   let proxyConfig = {proxyType: "manual"};
   caps = fromJSON({proxy: proxyConfig});
   equal("manual", caps.get("proxy").proxyType);
 
   let timeoutsConfig = {implicit: 123};
   caps = fromJSON({timeouts: timeoutsConfig});
   equal(123, caps.get("timeouts").implicit);
 
-  equal(0, caps.get("specificationLevel"));
-  caps = fromJSON({specificationLevel: 123});
-  equal(123, caps.get("specificationLevel"));
-  Assert.throws(() => fromJSON({specificationLevel: "foo"}));
-  Assert.throws(() => fromJSON({specificationLevel: -1}));
-
   caps = fromJSON({"moz:accessibilityChecks": true});
   equal(true, caps.get("moz:accessibilityChecks"));
   caps = fromJSON({"moz:accessibilityChecks": false});
   equal(false, caps.get("moz:accessibilityChecks"));
   Assert.throws(() => fromJSON({"moz:accessibilityChecks": "foo"}));
 
   run_next_test();
 });