diff --git a/dom/media/eme/MediaKeySession.cpp b/dom/media/eme/MediaKeySession.cpp
index 4b852dbc9946..2bc63e1f2849 100644
--- a/dom/media/eme/MediaKeySession.cpp
+++ b/dom/media/eme/MediaKeySession.cpp
@@ -209,11 +209,12 @@ MediaKeySession::GenerateRequest(const nsAString& aInitDataType,
                                       aInitDataType, data);
 
   EME_LOG("MediaKeySession[%p,'%s'] GenerateRequest() sent, "
-          "promiseId=%d initData(base64)='%s'",
+          "promiseId=%d initData(base64)='%s' initDataType='%s'",
           this,
           NS_ConvertUTF16toUTF8(mSessionId).get(),
           pid,
-          base64InitData.get());
+          base64InitData.get(),
+          NS_ConvertUTF16toUTF8(aInitDataType).get());
 
   return promise.forget();
 }
diff --git a/dom/media/eme/MediaKeySystemAccess.cpp b/dom/media/eme/MediaKeySystemAccess.cpp
index 4494f400742e..e66df5b3f25c 100644
--- a/dom/media/eme/MediaKeySystemAccess.cpp
+++ b/dom/media/eme/MediaKeySystemAccess.cpp
@@ -441,9 +441,12 @@ GetSupportedConfig(mozIGeckoMediaPluginService* aGMPService,
   if (aCandidate.mInitDataTypes.WasPassed()) {
     nsTArray<nsString> initDataTypes;
     for (const nsString& candidate : aCandidate.mInitDataTypes.Value()) {
+      // All supported keySystems can handle "cenc" initDataType.
+      // ClearKey also supports "keyids" and "webm" initDataTypes.
       if (candidate.EqualsLiteral("cenc")) {
         initDataTypes.AppendElement(candidate);
-      } else if (candidate.EqualsLiteral("keyids") &&
+      } else if ((candidate.EqualsLiteral("keyids") ||
+                  candidate.EqualsLiteral("webm)")) &&
                  aKeySystem.EqualsLiteral("org.w3.clearkey")) {
         initDataTypes.AppendElement(candidate);
       }
diff --git a/media/gmp-clearkey/0.1/ClearKeySession.cpp b/media/gmp-clearkey/0.1/ClearKeySession.cpp
index 8dcad9afa1f1..f7d9ef646e64 100644
--- a/media/gmp-clearkey/0.1/ClearKeySession.cpp
+++ b/media/gmp-clearkey/0.1/ClearKeySession.cpp
@@ -69,6 +69,11 @@ ClearKeySession::Init(uint32_t aCreateSessionToken,
       mCallback->RejectPromise(aPromiseId, kGMPAbortError, message, strlen(message));
       return;
     }
+  } else if (aInitDataType == "webm" && aInitDataSize == 16) {
+    // "webm" initData format is simply the raw bytes of the keyId.
+    vector<uint8_t> keyId;
+    keyId.assign(aInitData, aInitData+aInitDataSize);
+    mKeyIds.push_back(keyId);
   }
 
   if (!mKeyIds.size()) {
diff --git a/media/gmp-clearkey/0.1/ClearKeySessionManager.cpp b/media/gmp-clearkey/0.1/ClearKeySessionManager.cpp
index 587a27b90d7f..e6d5bd07ebff 100644
--- a/media/gmp-clearkey/0.1/ClearKeySessionManager.cpp
+++ b/media/gmp-clearkey/0.1/ClearKeySessionManager.cpp
@@ -102,10 +102,13 @@ ClearKeySessionManager::CreateSession(uint32_t aCreateSessionToken,
   CK_LOGD("ClearKeySessionManager::CreateSession type:%s", aInitDataType);
 
   string initDataType(aInitDataType, aInitDataType + aInitDataTypeSize);
-  // initDataType must be "cenc" or "keyids".
-  if (initDataType != "cenc" && initDataType != "keyids") {
+  // initDataType must be "cenc", "keyids", or "webm".
+  if (initDataType != "cenc" &&
+      initDataType != "keyids" &&
+      initDataType != "webm") {
+    string message = "'" + initDataType + "' is an initDataType unsupported by ClearKey";
     mCallback->RejectPromise(aPromiseId, kGMPNotSupportedError,
-                             nullptr /* message */, 0 /* messageLen */);
+                             message.c_str(), message.size());
     return;
   }