--- a/dom/workers/test/test_worker_interfaces.js
+++ b/dom/workers/test/test_worker_interfaces.js
@@ -71,21 +71,17 @@ var ecmaGlobals =
     {name: "TypeError", insecureContext: true},
     {name: "Uint16Array", insecureContext: true},
     {name: "Uint32Array", insecureContext: true},
     {name: "Uint8Array", insecureContext: true},
     {name: "Uint8ClampedArray", insecureContext: true},
     {name: "URIError", insecureContext: true},
     {name: "WeakMap", insecureContext: true},
     {name: "WeakSet", insecureContext: true},
-    // WebAssembly is not supported on some hardware configurations,
-    // but we have no way to check that from here.  Just give up for
-    // now and don't check for it at all.  Do NOT add any other uses
-    // of "optional"!
-    {name: "WebAssembly", insecureContext: true, optional: true},
+    {name: "WebAssembly", insecureContext: true, disabled: !getJSTestingFunctions().wasmIsSupportedByHardware()},
   ];
 // IMPORTANT: Do not change the list above without review from
 //            a JavaScript Engine peer!
 
 // IMPORTANT: Do not change the list below without review from a DOM peer!
 var interfaceNamesInGlobalScope =
   [
 // IMPORTANT: Do not change this list without review from a DOM peer!
@@ -286,18 +282,16 @@ function createInterfaceMap(version, use
 	    // The insecureContext test is very purposefully converting
 	    // entry.insecureContext to boolean, so undefined will convert to
 	    // false.  That way entries without an insecureContext annotation
 	    // will get treated as "insecureContext: false", which means exposed
 	    // only in secure contexts.
             (isInsecureContext && !Boolean(entry.insecureContext)) ||
             entry.disabled) {
           interfaceMap[entry.name] = false;
-        } else if (entry.optional) {
-          interfaceMap[entry.name] = "optional";
         } else {
           interfaceMap[entry.name] = true;
         }
       }
     }
   }
 
   addInterfaces(ecmaGlobals);
@@ -308,31 +302,27 @@ function createInterfaceMap(version, use
 
 function runTest(version, userAgent) {
   var interfaceMap = createInterfaceMap(version, userAgent);
   for (var name of Object.getOwnPropertyNames(self)) {
     // An interface name should start with an upper case character.
     if (!/^[A-Z]/.test(name)) {
       continue;
     }
-    ok(interfaceMap[name] === "optional" || interfaceMap[name],
+    ok(interfaceMap[name],
        "If this is failing: DANGER, are you sure you want to expose the new interface " + name +
        " to all webpages as a property on the worker? Do not make a change to this file without a " +
        " review from a DOM peer for that specific change!!! (or a JS peer for changes to ecmaGlobals)");
     delete interfaceMap[name];
   }
   for (var name of Object.keys(interfaceMap)) {
-    if (interfaceMap[name] === "optional") {
+    ok(name in self === interfaceMap[name],
+       name + " should " + (interfaceMap[name] ? "" : " NOT") + " be defined on the global scope");
+    if (!interfaceMap[name]) {
       delete interfaceMap[name];
-    } else {
-      ok(name in self === interfaceMap[name],
-         name + " should " + (interfaceMap[name] ? "" : " NOT") + " be defined on the global scope");
-      if (!interfaceMap[name]) {
-        delete interfaceMap[name];
-      }
     }
   }
   is(Object.keys(interfaceMap).length, 0,
      "The following interface(s) are not enumerated: " + Object.keys(interfaceMap).join(", "));
 }
 
 workerTestGetVersion(function(version) {
   workerTestGetUserAgent(function(userAgent) {