--- a/dom/base/nsTreeSanitizer.cpp
+++ b/dom/base/nsTreeSanitizer.cpp
@@ -1093,19 +1093,20 @@ nsTreeSanitizer::SanitizeStyleSheet(cons
   // Create a sheet to hold the parsed CSS
   RefPtr<StyleSheet> sheet =
     new ServoStyleSheet(mozilla::css::eAuthorSheetFeatures,
                         CORS_NONE, aDocument->GetReferrerPolicy(),
                         SRIMetadata());
   sheet->SetURIs(aDocument->GetDocumentURI(), nullptr, aBaseURI);
   sheet->SetPrincipal(aDocument->NodePrincipal());
   sheet->AsServo()->ParseSheetSync(
-    aDocument->CSSLoader(), NS_ConvertUTF16toUTF8(aOriginal),
-    aDocument->GetDocumentURI(), aBaseURI, aDocument->NodePrincipal(),
-    /* aLoadData = */ nullptr, 0, aDocument->GetCompatibilityMode());
+    aDocument->CSSLoader(),
+    NS_ConvertUTF16toUTF8(aOriginal),
+    /* aLoadData = */ nullptr,
+    /* aLineNumber = */ 0);
   NS_ENSURE_SUCCESS(rv, true);
   // Mark the sheet as complete.
   MOZ_ASSERT(!sheet->HasForcedUniqueInner(),
              "should not get a forced unique inner during parsing");
   sheet->SetComplete();
   // Loop through all the rules found in the CSS text
   ErrorResult err;
   RefPtr<dom::CSSRuleList> rules =

--- a/layout/style/Loader.cpp
+++ b/layout/style/Loader.cpp
@@ -1630,26 +1630,17 @@ Loader::DoParseSheetServo(ServoStyleShee
                           bool aAllowAsync,
                           bool& aCompleted)
 {
   aLoadData->mIsBeingParsed = true;
 
   // Some cases, like inline style and UA stylesheets, need to be parsed
   // synchronously. The former may trigger child loads, the latter must not.
   if (aLoadData->mSyncLoad || !aAllowAsync) {
-    aSheet->ParseSheetSync(
-      this,
-      aBytes,
-      aSheet->GetSheetURI(),
-      aSheet->GetBaseURI(),
-      aSheet->Principal(),
-      aLoadData,
-      aLoadData->mLineNumber,
-      GetCompatibilityMode()
-    );
+    aSheet->ParseSheetSync(this, aBytes, aLoadData, aLoadData->mLineNumber);
     aLoadData->mIsBeingParsed = false;
 
     bool noPendingChildren = aLoadData->mPendingChildren == 0;
     MOZ_ASSERT_IF(aLoadData->mSyncLoad, noPendingChildren);
     if (noPendingChildren) {
       aCompleted = true;
       SheetComplete(aLoadData, NS_OK);
     }
@@ -1659,37 +1650,28 @@ Loader::DoParseSheetServo(ServoStyleShee
 
   // This parse does not need to be synchronous. \o/
   //
   // Note that we need to block onload because there may be no network requests
   // pending.
   BlockOnload();
   RefPtr<SheetLoadData> loadData = aLoadData;
   nsCOMPtr<nsISerialEventTarget> target = DispatchTarget();
-  aSheet->ParseSheet(
-    this,
-    aBytes,
-    aSheet->GetSheetURI(),
-    aSheet->GetBaseURI(),
-    aSheet->Principal(),
-    aLoadData,
-    aLoadData->mLineNumber,
-    GetCompatibilityMode()
-  )->Then(target, __func__,
-          [loadData](bool aDummy) {
-            MOZ_ASSERT(NS_IsMainThread());
-            loadData->mIsBeingParsed = false;
-            loadData->mLoader->UnblockOnload(/* aFireSync = */ false);
-            // If there are no child sheets outstanding, mark us as complete.
-            // Otherwise, the children are holding strong refs to the data and
-            // will call SheetComplete() on it when they complete.
-            if (loadData->mPendingChildren == 0) {
-              loadData->mLoader->SheetComplete(loadData, NS_OK);
-            }
-          }, [] { MOZ_CRASH("rejected parse promise"); }
+  aSheet->ParseSheet(this, aBytes, aLoadData)->Then(target, __func__,
+    [loadData](bool aDummy) {
+      MOZ_ASSERT(NS_IsMainThread());
+      loadData->mIsBeingParsed = false;
+      loadData->mLoader->UnblockOnload(/* aFireSync = */ false);
+      // If there are no child sheets outstanding, mark us as complete.
+      // Otherwise, the children are holding strong refs to the data and
+      // will call SheetComplete() on it when they complete.
+      if (loadData->mPendingChildren == 0) {
+        loadData->mLoader->SheetComplete(loadData, NS_OK);
+      }
+    }, [] { MOZ_CRASH("rejected parse promise"); }
   );
 
   return NS_OK;
 }
 
 /**
  * SheetComplete is the do-it-all cleanup function.  It removes the
  * load data from the "loading" hashtable, adds the sheet to the

--- a/layout/style/ServoStyleSheet.cpp
+++ b/layout/style/ServoStyleSheet.cpp
@@ -229,49 +229,47 @@ AllowParallelParse(css::Loader* aLoader,
   }
 
   return true;
 }
 
 RefPtr<StyleSheetParsePromise>
 ServoStyleSheet::ParseSheet(css::Loader* aLoader,
                             const nsACString& aBytes,
-                            nsIURI* aSheetURI,
-                            nsIURI* aBaseURI,
-                            nsIPrincipal* aSheetPrincipal,
-                            css::SheetLoadData* aLoadData,
-                            uint32_t aLineNumber,
-                            nsCompatibility aCompatMode)
+                            css::SheetLoadData* aLoadData)
 {
+  MOZ_ASSERT(aLoader);
+  MOZ_ASSERT(aLoadData);
   MOZ_ASSERT(mParsePromise.IsEmpty());
   RefPtr<StyleSheetParsePromise> p = mParsePromise.Ensure(__func__);
-  Inner()->mURLData = new URLExtraData(aBaseURI, aSheetURI, aSheetPrincipal); // RefPtr
+  Inner()->mURLData =
+    new URLExtraData(GetBaseURI(), GetSheetURI(), Principal()); // RefPtr
 
-  if (!AllowParallelParse(aLoader, aSheetURI)) {
+  if (!AllowParallelParse(aLoader, GetSheetURI())) {
     RefPtr<RawServoStyleSheetContents> contents =
       Servo_StyleSheet_FromUTF8Bytes(aLoader,
                                      this,
                                      aLoadData,
                                      &aBytes,
                                      mParsingMode,
                                      Inner()->mURLData,
-                                     aLineNumber,
-                                     aCompatMode,
+                                     aLoadData->mLineNumber,
+                                     aLoader->GetCompatibilityMode(),
                                      /* reusable_sheets = */ nullptr)
       .Consume();
     FinishAsyncParse(contents.forget());
   } else {
     RefPtr<css::SheetLoadDataHolder> loadDataHolder =
       new css::SheetLoadDataHolder(__func__, aLoadData);
     Servo_StyleSheet_FromUTF8BytesAsync(loadDataHolder,
                                         Inner()->mURLData,
                                         &aBytes,
                                         mParsingMode,
-                                        aLineNumber,
-                                        aCompatMode);
+                                        aLoadData->mLineNumber,
+                                        aLoader->GetCompatibilityMode());
   }
 
   return Move(p);
 }
 
 void
 ServoStyleSheet::FinishAsyncParse(already_AddRefed<RawServoStyleSheetContents> aSheetContents)
 {
@@ -281,34 +279,32 @@ ServoStyleSheet::FinishAsyncParse(alread
   FinishParse();
   mParsePromise.Resolve(true, __func__);
 }
 
 
 void
 ServoStyleSheet::ParseSheetSync(css::Loader* aLoader,
                                 const nsACString& aBytes,
-                                nsIURI* aSheetURI,
-                                nsIURI* aBaseURI,
-                                nsIPrincipal* aSheetPrincipal,
                                 css::SheetLoadData* aLoadData,
                                 uint32_t aLineNumber,
-                                nsCompatibility aCompatMode,
                                 css::LoaderReusableStyleSheets* aReusableSheets)
 {
-  Inner()->mURLData = new URLExtraData(aBaseURI, aSheetURI, aSheetPrincipal); // RefPtr
+  nsCompatibility compatMode =
+    aLoader ? aLoader->GetCompatibilityMode() : eCompatibility_FullStandards;
 
+  Inner()->mURLData = new URLExtraData(GetBaseURI(), GetSheetURI(), Principal()); // RefPtr
   Inner()->mContents = Servo_StyleSheet_FromUTF8Bytes(aLoader,
                                                       this,
                                                       aLoadData,
                                                       &aBytes,
                                                       mParsingMode,
                                                       Inner()->mURLData,
                                                       aLineNumber,
-                                                      aCompatMode,
+                                                      compatMode,
                                                       aReusableSheets)
                          .Consume();
 
   FinishParse();
 }
 
 void
 ServoStyleSheet::FinishParse()
@@ -385,22 +381,18 @@ ServoStyleSheet::ReparseSheet(const nsAS
       RuleRemoved(*rule);
     }
   }
 
   DropRuleList();
 
   ParseSheetSync(loader,
                  NS_ConvertUTF16toUTF8(aInput),
-                 mInner->mSheetURI,
-                 mInner->mBaseURI,
-                 mInner->mPrincipal,
                  /* aLoadData = */ nullptr,
                  lineNumber,
-                 eCompatibility_FullStandards,
                  &reusableSheets);
   DidDirty();
 
   // Notify the stylesets about the new rules.
   {
     // Get the rule list (which will need to be regenerated after ParseSheet).
     ServoCSSRuleList* ruleList = GetCssRulesInternal();
     MOZ_ASSERT(ruleList);

--- a/layout/style/ServoStyleSheet.h
+++ b/layout/style/ServoStyleSheet.h
@@ -89,38 +89,29 @@ public:
 
   bool HasRules() const;
 
   // Parses a stylesheet. The aLoadData argument corresponds to the
   // SheetLoadData for this stylesheet. It may be null in some cases.
   RefPtr<StyleSheetParsePromise>
   ParseSheet(css::Loader* aLoader,
              const nsACString& aBytes,
-             nsIURI* aSheetURI,
-             nsIURI* aBaseURI,
-             nsIPrincipal* aSheetPrincipal,
-             css::SheetLoadData* aLoadData,
-             uint32_t aLineNumber,
-             nsCompatibility aCompatMode);
+             css::SheetLoadData* aLoadData);
 
   // Common code that needs to be called after servo finishes parsing. This is
   // shared between the parallel and sequential paths.
   void FinishAsyncParse(already_AddRefed<RawServoStyleSheetContents> aSheetContents);
 
   // Similar to the above, but guarantees that parsing will be performed
   // synchronously.
   void
   ParseSheetSync(css::Loader* aLoader,
                  const nsACString& aBytes,
-                 nsIURI* aSheetURI,
-                 nsIURI* aBaseURI,
-                 nsIPrincipal* aSheetPrincipal,
                  css::SheetLoadData* aLoadData,
                  uint32_t aLineNumber,
-                 nsCompatibility aCompatMode,
                  css::LoaderReusableStyleSheets* aReusableSheets = nullptr);
 
   nsresult ReparseSheet(const nsAString& aInput);
 
   const RawServoStyleSheetContents* RawContents() const {
     return Inner()->mContents;
   }
 

--- a/layout/style/nsLayoutStylesheetCache.cpp
+++ b/layout/style/nsLayoutStylesheetCache.cpp
@@ -883,20 +883,21 @@ nsLayoutStylesheetCache::BuildPreference
         NS_GET_R_G_B(focusBG));
   }
 
   NS_ASSERTION(sheetText.Length() <= kPreallocSize,
                "kPreallocSize should be big enough to build preference style "
                "sheet without reallocation");
 
   ServoStyleSheet* servoSheet = sheet->AsServo();
-  // NB: The pref sheet never has @import rules.
-  servoSheet->ParseSheetSync(nullptr, sheetText, uri, uri, nullptr,
-                             /* aLoadData = */ nullptr, 0,
-                             eCompatibility_FullStandards);
+  // NB: The pref sheet never has @import rules, thus no loader.
+  servoSheet->ParseSheetSync(nullptr,
+                             sheetText,
+                             /* aLoadData = */ nullptr,
+                             /* aLineNumber = */ 0);
 
 #undef NS_GET_R_G_B
 }
 
 mozilla::StaticRefPtr<nsLayoutStylesheetCache>
 nsLayoutStylesheetCache::gStyleCache;
 
 mozilla::StaticRefPtr<mozilla::css::Loader>