--- a/dom/webidl/StyleSheet.webidl
+++ b/dom/webidl/StyleSheet.webidl
@@ -19,16 +19,24 @@ interface StyleSheet {
   [Pure]
   readonly attribute StyleSheet? parentStyleSheet;
   [Pure]
   readonly attribute DOMString? title;
   [Constant]
   readonly attribute MediaList media;
   [Pure]
   attribute boolean disabled;
+  // The source map URL for this style sheet.  The source map URL can
+  // be found in one of two ways.
+  //
   // If a SourceMap or X-SourceMap response header is seen, this is
-  // the value.  If both are seen, SourceMap is preferred.  If neither
-  // is seen, this will be an empty string.  Because this relies on
-  // the HTTP response, it can change if checked before the response
-  // is available -- which is why it is not [Constant].
+  // the value.  If both are seen, SourceMap is preferred.  Because
+  // this relies on the HTTP response, it can change if checked before
+  // the response is available -- which is why it is not [Constant].
+  //
+  // If the style sheet has the special "# sourceMappingURL=" comment,
+  // then this is the URL specified there.
+  //
+  // If the source map URL is not found by either of these methods,
+  // then this is an empty string.
   [ChromeOnly, Pure]
   readonly attribute DOMString sourceMapURL;
 };

--- a/layout/style/ServoBindingList.h
+++ b/layout/style/ServoBindingList.h
@@ -56,16 +56,18 @@ SERVO_BINDING_FUNC(Servo_StyleSheet_HasR
 SERVO_BINDING_FUNC(Servo_StyleSheet_GetRules, ServoCssRulesStrong,
                    RawServoStyleSheetContentsBorrowed sheet)
 SERVO_BINDING_FUNC(Servo_StyleSheet_Clone, RawServoStyleSheetContentsStrong,
                    RawServoStyleSheetContentsBorrowed sheet,
                    const mozilla::ServoStyleSheet* reference_sheet);
 SERVO_BINDING_FUNC(Servo_StyleSheet_SizeOfIncludingThis, size_t,
                    mozilla::MallocSizeOf malloc_size_of,
                    RawServoStyleSheetContentsBorrowed sheet)
+SERVO_BINDING_FUNC(Servo_StyleSheet_GetSourceMapURL, void,
+                   RawServoStyleSheetContentsBorrowed sheet, nsAString* result)
 // We'd like to return `OriginFlags` here, but bindgen bitfield enums don't
 // work as return values with the Linux 32-bit ABI at the moment because
 // they wrap the value in a struct.
 SERVO_BINDING_FUNC(Servo_StyleSheet_GetOrigin, uint8_t,
                    RawServoStyleSheetContentsBorrowed sheet)
 SERVO_BINDING_FUNC(Servo_StyleSet_Init, RawServoStyleSet*, RawGeckoPresContextOwned pres_context)
 SERVO_BINDING_FUNC(Servo_StyleSet_RebuildCachedData, void,
                    RawServoStyleSetBorrowed set)

--- a/layout/style/ServoStyleSheet.cpp
+++ b/layout/style/ServoStyleSheet.cpp
@@ -213,16 +213,20 @@ ServoStyleSheet::ParseSheet(css::Loader*
                                                       aInput.Length(),
                                                       mParsingMode,
                                                       extraData,
                                                       aLineNumber,
                                                       aCompatMode,
                                                       aReusableSheets)
                          .Consume();
 
+  nsString sourceMapURL;
+  Servo_StyleSheet_GetSourceMapURL(Inner()->mContents, &sourceMapURL);
+  SetSourceMapURLFromComment(sourceMapURL);
+
   Inner()->mURLData = extraData.forget();
   return NS_OK;
 }
 
 nsresult
 ServoStyleSheet::ReparseSheet(const nsAString& aInput)
 {
   if (!mInner->mComplete) {

--- a/layout/style/StyleSheet.cpp
+++ b/layout/style/StyleSheet.cpp
@@ -255,16 +255,17 @@ StyleSheetInfo::StyleSheetInfo(StyleShee
   , mPrincipal(aCopy.mPrincipal)
   , mCORSMode(aCopy.mCORSMode)
   , mReferrerPolicy(aCopy.mReferrerPolicy)
   , mIntegrity(aCopy.mIntegrity)
   , mComplete(aCopy.mComplete)
   , mFirstChild()  // We don't rebuild the child because we're making a copy
                    // without children.
   , mSourceMapURL(aCopy.mSourceMapURL)
+  , mSourceMapURLFromComment(aCopy.mSourceMapURLFromComment)
 #ifdef DEBUG
   , mPrincipalSet(aCopy.mPrincipalSet)
 #endif
 {
   AddSheet(aPrimarySheet);
 }
 
 StyleSheetInfo::~StyleSheetInfo()
@@ -505,25 +506,35 @@ StyleSheet::GetCssRules(nsIPrincipal& aS
     return nullptr;
   }
   FORWARD_INTERNAL(GetCssRulesInternal, ())
 }
 
 void
 StyleSheet::GetSourceMapURL(nsAString& aSourceMapURL)
 {
-  aSourceMapURL = mInner->mSourceMapURL;
+  if (mInner->mSourceMapURL.IsEmpty()) {
+    aSourceMapURL = mInner->mSourceMapURLFromComment;
+  } else {
+    aSourceMapURL = mInner->mSourceMapURL;
+  }
 }
 
 void
 StyleSheet::SetSourceMapURL(const nsAString& aSourceMapURL)
 {
   mInner->mSourceMapURL = aSourceMapURL;
 }
 
+void
+StyleSheet::SetSourceMapURLFromComment(const nsAString& aSourceMapURLFromComment)
+{
+  mInner->mSourceMapURLFromComment = aSourceMapURLFromComment;
+}
+
 css::Rule*
 StyleSheet::GetDOMOwnerRule() const
 {
   return mOwnerRule;
 }
 
 uint32_t
 StyleSheet::InsertRule(const nsAString& aRule, uint32_t aIndex,

--- a/layout/style/StyleSheet.h
+++ b/layout/style/StyleSheet.h
@@ -210,16 +210,17 @@ public:
   // GetOwnerNode is defined above.
   inline StyleSheet* GetParentStyleSheet() const;
   // The XPCOM GetTitle is fine for WebIDL.
   dom::MediaList* Media();
   bool Disabled() const { return mDisabled; }
   // The XPCOM SetDisabled is fine for WebIDL.
   void GetSourceMapURL(nsAString& aTitle);
   void SetSourceMapURL(const nsAString& aSourceMapURL);
+  void SetSourceMapURLFromComment(const nsAString& aSourceMapURLFromComment);
 
   // WebIDL CSSStyleSheet API
   // Can't be inline because we can't include ImportRule here.  And can't be
   // called GetOwnerRule because that would be ambiguous with the ImportRule
   // version.
   css::Rule* GetDOMOwnerRule() const;
   dom::CSSRuleList* GetCssRules(nsIPrincipal& aSubjectPrincipal,
                                 ErrorResult& aRv);

--- a/layout/style/StyleSheetInfo.h
+++ b/layout/style/StyleSheetInfo.h
@@ -60,16 +60,21 @@ struct StyleSheetInfo
   // throughout its parent chain and things are good.
   RefPtr<StyleSheet>     mFirstChild;
   AutoTArray<StyleSheet*, 8> mSheets;
 
   // If a SourceMap or X-SourceMap response header is seen, this is
   // the value.  If both are seen, SourceMap is preferred.  If neither
   // is seen, this will be an empty string.
   nsString mSourceMapURL;
+  // This stores any source map URL that might have been seen in a
+  // comment in the style sheet.  This is separate from mSourceMapURL
+  // so that the value does not overwrite any value that might have
+  // come from a response header.
+  nsString mSourceMapURLFromComment;
 
 #ifdef DEBUG
   bool                   mPrincipalSet;
 #endif
 };
 
 } // namespace mozilla
 

--- a/layout/style/nsCSSParser.cpp
+++ b/layout/style/nsCSSParser.cpp
@@ -1641,16 +1641,18 @@ CSSParserImpl::ParseSheet(const nsAStrin
       ParseAtRule(AppendRuleToSheet, this, false);
       continue;
     }
     UngetToken();
     if (ParseRuleSet(AppendRuleToSheet, this)) {
       mSection = eCSSSection_General;
     }
   }
+
+  mSheet->SetSourceMapURLFromComment(scanner.GetSourceMapURL());
   ReleaseScanner();
 
   mParsingMode = css::eAuthorSheetFeatures;
   mIsChrome = false;
   mReusableSheets = nullptr;
 
   return NS_OK;
 }

--- a/layout/style/nsCSSScanner.cpp
+++ b/layout/style/nsCSSScanner.cpp
@@ -543,42 +543,88 @@ nsCSSScanner::SkipWhitespace()
 }
 
 /**
  * Skip over one CSS comment starting at the current read position.
  */
 void
 nsCSSScanner::SkipComment()
 {
+  static const char sourceMappingURLDirective[] = "# sourceMappingURL=";
+
   MOZ_ASSERT(Peek() == '/' && Peek(1) == '*', "should not have been called");
   Advance(2);
+  // Look in each comment for a source map directive; using a simple
+  // state machine.  The states are:
+  // * sourceMapIndex >= 0 means that we're still looking for the
+  //   directive and expect the next character to be at that index of
+  //   sourceMappingURLDirective.
+  //   As a special case, when sourceMapIndex == 0, '@' is also recognized.
+  // * sourceMapIndex < 0 means that we don't need to look for the
+  //   directive any more -- whether it was found or not.
+  // * copying == true means that the directive was found and we're
+  //   copying characters into mSourceMapURL.  This stops at the first
+  //   whitespace, or at the end of the comment.
+  int sourceMapIndex = 0;
+  bool copying = false;
   for (;;) {
     int32_t ch = Peek();
     if (ch < 0) {
       if (mReporter)
         mReporter->ReportUnexpectedEOF("PECommentEOF");
       SetEOFCharacters(eEOFCharacters_Asterisk | eEOFCharacters_Slash);
       return;
     }
+    if (sourceMapIndex >= 0) {
+      if ((sourceMapIndex == 0 && ch == '@') || ch == sourceMappingURLDirective[sourceMapIndex]) {
+        ++sourceMapIndex;
+        if (sourceMappingURLDirective[sourceMapIndex] == '\0') {
+          sourceMapIndex = -1;
+          mSourceMapURL.Truncate();
+          copying = true;
+          Advance();
+          // Make sure we don't copy out the '=' by falling through.
+          continue;
+        }
+      } else {
+        // Did not see the directive.
+        sourceMapIndex = -1;
+      }
+    }
+
     if (ch == '*') {
       Advance();
       ch = Peek();
       if (ch < 0) {
+        // In this case, even if we saw a source map directive, leave
+        // the "*" out of it.
         if (mReporter)
           mReporter->ReportUnexpectedEOF("PECommentEOF");
         SetEOFCharacters(eEOFCharacters_Slash);
         return;
       }
       if (ch == '/') {
         Advance();
         return;
       }
+      if (copying) {
+        mSourceMapURL.Append('*');
+      }
     } else if (IsVertSpace(ch)) {
       AdvanceLine();
+      // Done with the directive, so stop copying.
+      copying = false;
+    } else if (IsWhitespace(ch)) {
+      Advance();
+      // Done with the directive, so stop copying.
+      copying = false;
     } else {
+      if (copying) {
+        mSourceMapURL.Append(ch);
+      }
       Advance();
     }
   }
 }
 
 /**
  * If there is a valid escape sequence starting at the current read
  * position, consume it, decode it, append the result to |aOutput|,

--- a/layout/style/nsCSSScanner.h
+++ b/layout/style/nsCSSScanner.h
@@ -227,16 +227,19 @@ class nsCSSScanner {
   { return mTokenOffset - mTokenLineOffset; }
 
   uint32_t GetTokenOffset() const
   { return mTokenOffset; }
 
   uint32_t GetTokenEndOffset() const
   { return mOffset; }
 
+  const nsAString& GetSourceMapURL() const
+  { return mSourceMapURL; }
+
   // Get the text of the line containing the first character of
   // the most recently processed token.
   nsDependentSubstring GetCurrentLine() const;
 
   // Get the next token.  Return false on EOF.  aTokenResult is filled
   // in with the data for the token.  aSkip controls whether
   // whitespace and/or comment tokens are ever returned.
   bool Next(nsCSSToken& aTokenResult, nsCSSScannerExclude aSkip);
@@ -356,16 +359,18 @@ protected:
   uint32_t mRecordStartOffset;
   EOFCharacters mEOFCharacters;
 
   mozilla::css::ErrorReporter *mReporter;
 
   bool mRecording;
   bool mSeenBadToken;
   bool mSeenVariableReference;
+
+  nsString mSourceMapURL;
 };
 
 // Token for the grid-template-areas micro-syntax
 // http://dev.w3.org/csswg/css-grid/#propdef-grid-template-areas
 struct MOZ_STACK_CLASS nsCSSGridTemplateAreaToken {
   nsAutoString mName;  // Empty for a null cell, non-empty for a named cell
   bool isTrash;  // True for a trash token, mName is ignored in this case.
 };

--- a/layout/style/test/browser.ini
+++ b/layout/style/test/browser.ini
@@ -8,8 +8,9 @@ support-files =
   mapped2.css
   mapped2.css^headers^
   sourcemap_css.html
 
 [browser_bug453896.js]
 [browser_newtab_share_rule_processors.js]
 skip-if = stylo # Gecko-specific test
 [browser_sourcemap.js]
+[browser_sourcemap_comment.js]

new file mode 100644
--- /dev/null
+++ b/layout/style/test/browser_sourcemap_comment.js
@@ -0,0 +1,40 @@
+add_task(async function() {
+  // Test text and expected results.
+  let test_cases = [
+    ["/*# sourceMappingURL=here*/", "here"],
+    ["/*# sourceMappingURL=here  */", "here"],
+    ["/*@ sourceMappingURL=here*/", "here"],
+    ["/*@ sourceMappingURL=there*/ /*# sourceMappingURL=here*/", "here"],
+    ["/*# sourceMappingURL=here there  */", "here"],
+
+    ["/*# sourceMappingURL=  here  */", ""],
+    ["/*# sourceMappingURL=*/", ""],
+    ["/*# sourceMappingUR=here  */", ""],
+    ["/*! sourceMappingURL=here  */", ""],
+    ["/*# sourceMappingURL = here  */", ""],
+    ["/*   # sourceMappingURL=here   */", ""],
+  ];
+
+  let page = "<!DOCTYPE HTML>\n<html>\n<head>\n";
+  for (let i = 0; i < test_cases.length; ++i) {
+    page += `<style type="text/css"> #x${i} { color: red; }${test_cases[i][0]}</style>\n`;
+  }
+  page += "</head><body>some text</body></html>";
+
+  let uri = "data:text/html;base64," + btoa(page);
+  info(`URI is ${uri}`);
+
+  await BrowserTestUtils.withNewTab({
+    gBrowser,
+    url: uri
+  }, async function(browser) {
+    await ContentTask.spawn(browser, test_cases, function* (tests) {
+      for (let i = 0; i < content.document.styleSheets.length; ++i) {
+        let sheet = content.document.styleSheets[i];
+
+        info(`Checking sheet #${i}`);
+        is(sheet.sourceMapURL, tests[i][1], `correct source map for sheet ${i}`);
+      }
+    });
+  });
+});

--- a/layout/style/test/mapped2.css
+++ b/layout/style/test/mapped2.css
@@ -1,3 +1,4 @@
 span {
   color: #f06;
 }
+//# sourceMappingURL: overridden-by-headers