--- a/devtools/client/framework/target.js
+++ b/devtools/client/framework/target.js
@@ -717,20 +717,44 @@ TabTarget.prototype = {
    * Log an error of some kind to the tab's console.
    *
    * @param {String} text
    *                 The text to log.
    * @param {String} category
    *                 The category of the message.  @see nsIScriptError.
    */
   logErrorInPage: function (text, category) {
-    if (this.activeTab && this.activeTab.traits.logErrorInPage) {
+    if (this.activeTab && this.activeTab.traits.logInPage) {
+      const errorFlag = 0;
       let packet = {
         to: this.form.actor,
-        type: "logErrorInPage",
+        type: "logInPage",
+        flags: errorFlag,
+        text,
+        category,
+      };
+      this.client.request(packet);
+    }
+  },
+
+  /**
+   * Log a warning of some kind to the tab's console.
+   *
+   * @param {String} text
+   *                 The text to log.
+   * @param {String} category
+   *                 The category of the message.  @see nsIScriptError.
+   */
+  logWarningInPage: function (text, category) {
+    if (this.activeTab && this.activeTab.traits.logInPage) {
+      const warningFlag = 1;
+      let packet = {
+        to: this.form.actor,
+        type: "logInPage",
+        flags: warningFlag,
         text,
         category,
       };
       this.client.request(packet);
     }
   },
 };
 
@@ -882,9 +906,13 @@ WorkerTarget.prototype = {
 
   makeRemote: function () {
     return Promise.resolve();
   },
 
   logErrorInPage: function () {
     // No-op.  See bug 1368680.
   },
+
+  logWarningInPage: function () {
+    // No-op.  See bug 1368680.
+  },
 };

--- a/devtools/client/framework/toolbox.js
+++ b/devtools/client/framework/toolbox.js
@@ -604,30 +604,30 @@ Toolbox.prototype = {
         switch (name) {
           case "getOriginalURLs":
             return (urlInfo) => {
               return target.getOriginalURLs(urlInfo)
                 .catch(text => {
                   let message = L10N.getFormatStr("toolbox.sourceMapFailure",
                                                   text, urlInfo.url,
                                                   urlInfo.sourceMapURL);
-                  this.target.logErrorInPage(message, "source map");
+                  this.target.logWarningInPage(message, "source map");
                   // It's ok to swallow errors here, because a null
                   // result just means that no source map was found.
                   return null;
                 });
             };
 
           case "getOriginalSourceText":
             return (originalSource) => {
               return target.getOriginalSourceText(originalSource)
                 .catch(text => {
                   let message = L10N.getFormatStr("toolbox.sourceMapSourceFailure",
                                                   text, originalSource.url);
-                  this.target.logErrorInPage(message, "source map");
+                  this.target.logWarningInPage(message, "source map");
                   // Also replace the result with the error text.
                   // Note that this result has to have the same form
                   // as whatever the upstream getOriginalSourceText
                   // returns.
                   return {
                     text: message,
                     contentType: "text/plain",
                   };
@@ -3077,17 +3077,17 @@ Toolbox.prototype = {
    *        as first argument.
    */
   addRequestFinishedListener: function (listener) {
     // Log console message informing the extension developer
     // that the Network panel needs to be selected at least
     // once in order to receive `onRequestFinished` events.
     let message = "The Network panel needs to be selected at least" +
       " once in order to receive 'onRequestFinished' events.";
-    this.target.logErrorInPage(message, "har");
+    this.target.logWarningInPage(message, "har");
 
     // Add the listener into internal list.
     this._requestFinishedListeners.add(listener);
   },
 
   removeRequestFinishedListener: function (listener) {
     this._requestFinishedListeners.delete(listener);
   },

--- a/devtools/client/webconsole/new-console-output/test/mochitest/browser.ini
+++ b/devtools/client/webconsole/new-console-output/test/mochitest/browser.ini
@@ -304,16 +304,17 @@ skip-if = true #	Bug 1405343
 [browser_webconsole_inspect_cross_domain_object.js]
 [browser_webconsole_js_input_expansion.js]
 [browser_webconsole_keyboard_accessibility.js]
 [browser_webconsole_location_debugger_link.js]
 [browser_webconsole_location_scratchpad_link.js]
 [browser_webconsole_location_styleeditor_link.js]
 [browser_webconsole_logErrorInPage.js]
 [browser_webconsole_loglimit.js]
+[browser_webconsole_logWarningInPage.js]
 [browser_webconsole_longstring_expand.js]
 skip-if = true #	Bug 1403448
 [browser_webconsole_longstring_hang.js]
 skip-if = true #	Bug 1403448
 [browser_webconsole_message_categories.js]
 [browser_webconsole_multiple_windows_and_tabs.js]
 [browser_webconsole_network_attach.js]
 [browser_webconsole_network_exceptions.js]

--- a/devtools/client/webconsole/new-console-output/test/mochitest/browser_webconsole_logErrorInPage.js
+++ b/devtools/client/webconsole/new-console-output/test/mochitest/browser_webconsole_logErrorInPage.js
@@ -12,9 +12,10 @@ const TEST_URI = "data:text/html;charset
 add_task(async function () {
   const hud = await openNewTabAndConsole(TEST_URI);
   const toolbox = hud.ui.newConsoleOutput.toolbox;
 
   toolbox.target.logErrorInPage("beware the octopus", "content javascript");
 
   const node = await waitFor(() => findMessage(hud, "octopus"));
   ok(node, "text is displayed in web console");
+  ok(node.classList.contains("error"), "the log represents an error");
 });

new file mode 100644
--- /dev/null
+++ b/devtools/client/webconsole/new-console-output/test/mochitest/browser_webconsole_logWarningInPage.js
@@ -0,0 +1,21 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set ft=javascript ts=2 et sw=2 tw=80: */
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that we can log warning message to the web console from the toolbox.
+
+const TEST_URI = "data:text/html;charset=utf-8,<p>test logErrorInPage";
+
+add_task(async function () {
+  const hud = await openNewTabAndConsole(TEST_URI);
+  const toolbox = hud.ui.newConsoleOutput.toolbox;
+
+  toolbox.target.logWarningInPage("beware the octopus", "content javascript");
+
+  const node = await waitFor(() => findMessage(hud, "octopus"));
+  ok(node, "text is displayed in web console");
+  ok(node.classList.contains("warn"), "the log represents a warning");
+});

--- a/devtools/client/webconsole/new-webconsole.js
+++ b/devtools/client/webconsole/new-webconsole.js
@@ -127,17 +127,17 @@ NewWebConsoleFrame.prototype = {
     return this._destroyer.promise;
   },
 
   _onUpdateListeners() {
 
   },
 
   logWarningAboutReplacedAPI() {
-    this.owner.target.logErrorInPage(l10n.getStr("ConsoleAPIDisabled"),
+    this.owner.target.logWarningInPage(l10n.getStr("ConsoleAPIDisabled"),
       "ConsoleAPIDisabled");
   },
 
   handleNetworkEventUpdate() {
 
   },
 
   /**

--- a/devtools/server/actors/tab.js
+++ b/devtools/server/actors/tab.js
@@ -223,18 +223,18 @@ function TabActor(connection) {
   this.traits = {
     reconfigure: true,
     // Supports frame listing via `listFrames` request and `frameUpdate` events
     // as well as frame switching via `switchToFrame` request
     frames: true,
     // Do not require to send reconfigure request to reset the document state
     // to what it was before using the TabActor
     noTabReconfigureOnClose: true,
-    // Supports the logErrorInPage request.
-    logErrorInPage: true,
+    // Supports the logInPage request.
+    logInPage: true,
   };
 
   this._workerActorList = null;
   this._workerActorPool = null;
   this._onWorkerActorListChanged = this._onWorkerActorListChanged.bind(this);
 }
 
 TabActor.prototype = {
@@ -651,21 +651,21 @@ TabActor.prototype = {
 
       return {
         "from": this.actorID,
         "workers": actors.map((actor) => actor.form())
       };
     });
   },
 
-  onLogErrorInPage(request) {
-    let {text, category} = request;
+  onLogInPage(request) {
+    let {text, category, flags} = request;
     let scriptErrorClass = Cc["@mozilla.org/scripterror;1"];
     let scriptError = scriptErrorClass.createInstance(Ci.nsIScriptError);
-    scriptError.initWithWindowID(text, null, null, 0, 0, 1,
+    scriptError.initWithWindowID(text, null, null, 0, 0, flags,
                                  category, getInnerId(this.window));
     Services.console.logMessage(scriptError);
     return {};
   },
 
   _onWorkerActorListChanged() {
     this._workerActorList.onListChanged = null;
     this.conn.sendActorEvent(this.actorID, "workerListChanged");
@@ -1428,17 +1428,17 @@ TabActor.prototype.requestTypes = {
   "detach": TabActor.prototype.onDetach,
   "focus": TabActor.prototype.onFocus,
   "reload": TabActor.prototype.onReload,
   "navigateTo": TabActor.prototype.onNavigateTo,
   "reconfigure": TabActor.prototype.onReconfigure,
   "switchToFrame": TabActor.prototype.onSwitchToFrame,
   "listFrames": TabActor.prototype.onListFrames,
   "listWorkers": TabActor.prototype.onListWorkers,
-  "logErrorInPage": TabActor.prototype.onLogErrorInPage,
+  "logInPage": TabActor.prototype.onLogInPage,
 };
 
 exports.TabActor = TabActor;
 
 /**
  * The DebuggerProgressListener object is an nsIWebProgressListener which
  * handles onStateChange events for the inspected browser. If the user tries to
  * navigate away from a paused page, the listener makes sure that the debuggee