--- a/devtools/client/styleeditor/StyleEditorUI.jsm
+++ b/devtools/client/styleeditor/StyleEditorUI.jsm
@@ -611,29 +611,29 @@ StyleEditorUI.prototype = {
           this.emit("editor-selected", showEditor);
 
           // Is there any CSS coverage markup to include?
           let usage = yield csscoverage.getUsage(this._target);
           if (usage == null) {
             return;
           }
 
-          let href = csscoverage.sheetToUrl(showEditor.styleSheet);
-          let reportData = yield usage.createEditorReport(href);
+          let sheet = showEditor.styleSheet;
+          let {reports} = yield usage.createEditorReportForSheet(sheet);
 
           showEditor.removeAllUnusedRegions();
 
-          if (reportData.reports.length > 0) {
+          if (reports.length > 0) {
             // Only apply if this file isn't compressed. We detect a
             // compressed file if there are more rules than lines.
             let editorText = showEditor.sourceEditor.getText();
             let lineCount = editorText.split("\n").length;
             let ruleCount = showEditor.styleSheet.ruleCount;
             if (lineCount >= ruleCount) {
-              showEditor.addUnusedRegions(reportData.reports);
+              showEditor.addUnusedRegions(reports);
             } else {
               this.emit("error", { key: "error-compressed", level: "info" });
             }
           }
         }.bind(this)).then(null, e => console.error(e));
       }.bind(this)
     });
   },

--- a/devtools/server/actors/csscoverage.js
+++ b/devtools/server/actors/csscoverage.js
@@ -335,16 +335,28 @@ var CSSUsageActor = protocol.ActorClassW
 
       reports.push(ruleReport);
     }
 
     return { reports: reports };
   },
 
   /**
+   * Compute the stylesheet URL and delegate the report creation to createEditorReport.
+   * See createEditorReport documentation.
+   *
+   * @param {StyleSheetActor} stylesheetActor
+   *        the stylesheet actor for which the coverage report should be generated.
+   */
+  createEditorReportForSheet: function (stylesheetActor) {
+    let url = sheetToUrl(stylesheetActor.rawSheet);
+    return this.createEditorReport(url);
+  },
+
+  /**
    * Returns a JSONable structure designed for the page report which shows
    * the recommended changes to a page.
    *
    * "preload" means that a rule is used before the load event happens, which
    * means that the page could by optimized by placing it in a <style> element
    * at the top of the page, moving the <link> elements to the bottom.
    *
    * Example:
@@ -689,31 +701,26 @@ exports.SEL_ALL = [
   SEL_EXTERNAL, SEL_FORM, SEL_ELEMENT, SEL_STRUCTURAL, SEL_SEMI,
   SEL_COMBINING, SEL_MEDIA
 ].reduce(function (prev, curr) {
   return prev.concat(curr);
 }, []);
 
 /**
  * Find a URL for a given stylesheet
- * @param stylesheet {StyleSheet|StyleSheetActor}
+ * @param {StyleSheet} stylesheet raw stylesheet
  */
-const sheetToUrl = exports.sheetToUrl = function (stylesheet) {
+const sheetToUrl = function (stylesheet) {
   // For <link> elements
   if (stylesheet.href) {
     return stylesheet.href;
   }
 
   // For <style> elements
   if (stylesheet.ownerNode) {
     let document = stylesheet.ownerNode.ownerDocument;
     let sheets = [...document.querySelectorAll("style")];
     let index = sheets.indexOf(stylesheet.ownerNode);
     return getURL(document) + " → <style> index " + index;
   }
 
-  // When `stylesheet` is a StyleSheetActor, we don't have access to ownerNode
-  if (stylesheet.nodeHref) {
-    return stylesheet.nodeHref + " → <style> index " + stylesheet.styleSheetIndex;
-  }
-
   throw new Error("Unknown sheet source");
 };

--- a/devtools/shared/specs/csscoverage.js
+++ b/devtools/shared/specs/csscoverage.js
@@ -1,15 +1,17 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 "use strict";
 
 const {Arg, RetVal, generateActorSpec} = require("devtools/shared/protocol");
 
+require("devtools/shared/specs/stylesheets");
+
 const cssUsageSpec = generateActorSpec({
   typeName: "cssUsage",
 
   events: {
     "state-change": {
       type: "stateChange",
       stateChange: Arg(0, "json")
     }
@@ -21,16 +23,20 @@ const cssUsageSpec = generateActorSpec({
     },
     stop: {},
     toggle: {},
     oneshot: {},
     createEditorReport: {
       request: { url: Arg(0, "string") },
       response: { reports: RetVal("array:json") }
     },
+    createEditorReportForSheet: {
+      request: { url: Arg(0, "stylesheet") },
+      response: { reports: RetVal("array:json") }
+    },
     createPageReport: {
       response: RetVal("json")
     },
     _testOnlyVisitedPages: {
       response: { value: RetVal("array:string") }
     },
   },
 });