--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -5353,22 +5353,16 @@ already_AddRefed<AnonymousContent>
 nsIDocument::InsertAnonymousContent(Element& aElement, ErrorResult& aRv)
 {
   nsIPresShell* shell = GetShell();
   if (!shell || !shell->GetCanvasFrame()) {
     aRv.Throw(NS_ERROR_UNEXPECTED);
     return nullptr;
   }
 
-  // We're about to insert random content here that will be rendered. We're
-  // going to need more than svg.css here...
-  if (IsSVGDocument()) {
-    AsSVGDocument()->EnsureNonSVGUserAgentStyleSheetsLoaded();
-  }
-
   nsAutoScriptBlocker scriptBlocker;
   nsCOMPtr<Element> container = shell->GetCanvasFrame()
                                      ->GetCustomContentContainer();
   if (!container) {
     aRv.Throw(NS_ERROR_UNEXPECTED);
     return nullptr;
   }
 

--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -2012,17 +2012,23 @@ public:
     return mType == eXUL;
   }
   bool IsUnstyledDocument()
   {
     return IsLoadedAsData() || IsLoadedAsInteractiveData();
   }
   bool LoadsFullXULStyleSheetUpFront()
   {
-    return IsXULDocument() || AllowXULXBL();
+    if (IsXULDocument()) {
+      return true;
+    }
+    if (IsSVGDocument()) {
+      return false;
+    }
+    return AllowXULXBL();
   }
 
   bool IsScriptEnabled();
 
   bool IsTopLevelContentDocument() const { return mIsTopLevelContentDocument; }
   void SetIsTopLevelContentDocument(bool aIsTopLevelContentDocument)
   {
     mIsTopLevelContentDocument = aIsTopLevelContentDocument;

--- a/dom/svg/SVGDocument.cpp
+++ b/dom/svg/SVGDocument.cpp
@@ -25,141 +25,30 @@ using namespace mozilla::css;
 using namespace mozilla::dom;
 
 namespace mozilla {
 namespace dom {
 
 //----------------------------------------------------------------------
 // Implementation
 
-//----------------------------------------------------------------------
-// nsISupports methods:
-
-nsresult
-SVGDocument::InsertChildBefore(nsIContent* aKid, nsIContent* aBeforeThis,
-                               bool aNotify)
-{
-  if (aKid->IsElement() && !aKid->IsSVGElement()) {
-    // We can get here when well formed XML with a non-SVG root element is
-    // served with the SVG MIME type, for example. In that case we need to load
-    // the non-SVG UA sheets or else we can get bugs like bug 1016145.  Note
-    // that we have to do this _before_ the XMLDocument::InsertChildBefore,
-    // since that can try to construct frames, and we need to have the sheets
-    // loaded by then.
-    EnsureNonSVGUserAgentStyleSheetsLoaded();
-  }
-
-  return XMLDocument::InsertChildBefore(aKid, aBeforeThis, aNotify);
-}
-
 nsresult
 SVGDocument::Clone(mozilla::dom::NodeInfo *aNodeInfo, nsINode **aResult,
                    bool aPreallocateChildren) const
 {
   NS_ASSERTION(aNodeInfo->NodeInfoManager() == mNodeInfoManager,
                "Can't import this document into another document!");
 
   RefPtr<SVGDocument> clone = new SVGDocument();
   nsresult rv = CloneDocHelper(clone.get(), aPreallocateChildren);
   NS_ENSURE_SUCCESS(rv, rv);
 
   return CallQueryInterface(clone.get(), aResult);
 }
 
-void
-SVGDocument::EnsureNonSVGUserAgentStyleSheetsLoaded()
-{
-  if (mHasLoadedNonSVGUserAgentStyleSheets) {
-    return;
-  }
-
-  if (IsStaticDocument()) {
-    // If we're a static clone of a document, then
-    // nsIDocument::CreateStaticClone will handle cloning the original
-    // document's sheets, including the on-demand non-SVG UA sheets,
-    // for us.
-    return;
-  }
-
-  mHasLoadedNonSVGUserAgentStyleSheets = true;
-
-  if (IsBeingUsedAsImage()) {
-    // nsDocumentViewer::CreateStyleSet skipped loading all user-agent/user
-    // style sheets in this case, but we'll need B2G/Fennec's
-    // content.css. We could load all the sheets registered with the
-    // nsIStyleSheetService (and maybe we should) but most likely it isn't
-    // desirable or necessary for foreignObject in SVG-as-an-image. Instead we
-    // only load the "agent-style-sheets" that nsStyleSheetService::Init()
-    // pulls in from the category manager. That keeps memory use of
-    // SVG-as-an-image down.
-    //
-    // We do this before adding the other sheets below because
-    // EnsureOnDemandBuiltInUASheet prepends, and B2G/Fennec's/GeckoView's
-    // content.css must come after those UASheet() etc.
-    //
-    // FIXME(emilio, bug 1468133): We may already have loaded some of the other
-    // on-demand built-in UA sheets, including svg.css, so this looks somewhat
-    // bogus... Also, this should probably just use the stylesheet service which
-    // also has the right sheets cached and parsed here...
-    nsCOMPtr<nsICategoryManager> catMan =
-      do_GetService(NS_CATEGORYMANAGER_CONTRACTID);
-    if (catMan) {
-      nsCOMPtr<nsISimpleEnumerator> sheets;
-      catMan->EnumerateCategory("agent-style-sheets", getter_AddRefs(sheets));
-      if (sheets) {
-        bool hasMore;
-        while (NS_SUCCEEDED(sheets->HasMoreElements(&hasMore)) && hasMore) {
-          nsCOMPtr<nsISupports> sheet;
-          if (NS_FAILED(sheets->GetNext(getter_AddRefs(sheet))))
-            break;
-
-          nsCOMPtr<nsISupportsCString> icStr = do_QueryInterface(sheet);
-          MOZ_ASSERT(icStr,
-                     "category manager entries must be nsISupportsCStrings");
-
-          nsAutoCString name;
-          icStr->GetData(name);
-
-          nsCString spec;
-          catMan->GetCategoryEntry("agent-style-sheets", name.get(),
-                                   getter_Copies(spec));
-
-          mozilla::css::Loader* cssLoader = CSSLoader();
-          if (cssLoader->GetEnabled()) {
-            nsCOMPtr<nsIURI> uri;
-            NS_NewURI(getter_AddRefs(uri), spec);
-            if (uri) {
-              RefPtr<StyleSheet> sheet;
-              cssLoader->LoadSheetSync(uri,
-                                       mozilla::css::eAgentSheetFeatures,
-                                       true, &sheet);
-              if (sheet) {
-                EnsureOnDemandBuiltInUASheet(sheet);
-              }
-            }
-          }
-        }
-      }
-    }
-  }
-
-  auto cache = nsLayoutStylesheetCache::Singleton();
-
-  EnsureOnDemandBuiltInUASheet(cache->FormsSheet());
-  EnsureOnDemandBuiltInUASheet(cache->CounterStylesSheet());
-  EnsureOnDemandBuiltInUASheet(cache->HTMLSheet());
-  if (nsLayoutUtils::ShouldUseNoFramesSheet(this)) {
-    EnsureOnDemandBuiltInUASheet(cache->NoFramesSheet());
-  }
-  if (nsLayoutUtils::ShouldUseNoScriptSheet(this)) {
-    EnsureOnDemandBuiltInUASheet(cache->NoScriptSheet());
-  }
-  EnsureOnDemandBuiltInUASheet(cache->UASheet());
-}
-
 } // namespace dom
 } // namespace mozilla
 
 ////////////////////////////////////////////////////////////////////////
 // Exported creation functions
 
 nsresult
 NS_NewSVGDocument(nsIDocument** aInstancePtrResult)

--- a/dom/svg/SVGDocument.h
+++ b/dom/svg/SVGDocument.h
@@ -17,46 +17,37 @@ namespace mozilla {
 class SVGContextPaint;
 
 namespace dom {
 
 class SVGForeignObjectElement;
 
 class SVGDocument final : public XMLDocument
 {
-  friend class SVGForeignObjectElement; // To call EnsureNonSVGUserAgentStyleSheetsLoaded
-  friend class nsIDocument; // Same reason.
-
 public:
   SVGDocument()
     : XMLDocument("image/svg+xml")
-    , mHasLoadedNonSVGUserAgentStyleSheets(false)
   {
     mType = eSVG;
   }
 
-  virtual nsresult InsertChildBefore(nsIContent* aKid, nsIContent* aBeforeThis,
-                                     bool aNotify) override;
   virtual nsresult Clone(mozilla::dom::NodeInfo *aNodeInfo, nsINode **aResult,
                          bool aPreallocateChildren) const override;
 
   void SetCurrentContextPaint(const SVGContextPaint* aContextPaint)
   {
     mCurrentContextPaint = aContextPaint;
   }
 
   const SVGContextPaint* GetCurrentContextPaint() const
   {
     return mCurrentContextPaint;
   }
 
 private:
-  void EnsureNonSVGUserAgentStyleSheetsLoaded();
-
-  bool mHasLoadedNonSVGUserAgentStyleSheets;
 
   // This is maintained by AutoSetRestoreSVGContextPaint.
   const SVGContextPaint* mCurrentContextPaint = nullptr;
 };
 
 } // namespace dom
 } // namespace mozilla
 

--- a/dom/svg/SVGForeignObjectElement.cpp
+++ b/dom/svg/SVGForeignObjectElement.cpp
@@ -102,42 +102,16 @@ SVGForeignObjectElement::HasValidDimensi
          mLengthAttributes[ATTR_WIDTH].GetAnimValInSpecifiedUnits() > 0 &&
          mLengthAttributes[ATTR_HEIGHT].IsExplicitlySet() &&
          mLengthAttributes[ATTR_HEIGHT].GetAnimValInSpecifiedUnits() > 0;
 }
 
 //----------------------------------------------------------------------
 // nsIContent methods
 
-nsresult
-SVGForeignObjectElement::BindToTree(nsIDocument* aDocument,
-                                    nsIContent* aParent,
-                                    nsIContent* aBindingParent,
-                                    bool aCompileEventHandlers)
-{
-  nsresult rv = SVGGraphicsElement::BindToTree(aDocument, aParent,
-                                               aBindingParent,
-                                               aCompileEventHandlers);
-  NS_ENSURE_SUCCESS(rv, rv);
-
-  nsIDocument* doc = GetComposedDoc();
-  if (doc && doc->IsSVGDocument()) {
-    // We assume that we're going to have HTML content, so we ensure that the
-    // UA style sheets that nsDocumentViewer::CreateStyleSet skipped when
-    // it saw the document was an SVG document are loaded.
-    //
-    // We setup these style sheets during binding, not element construction,
-    // because elements can be moved from the document that creates them to
-    // another document.
-    doc->AsSVGDocument()->EnsureNonSVGUserAgentStyleSheetsLoaded();
-  }
-
-  return rv;
-}
-
 NS_IMETHODIMP_(bool)
 SVGForeignObjectElement::IsAttributeMapped(const nsAtom* name) const
 {
   static const MappedAttributeEntry* const map[] = {
     sFEFloodMap,
     sFiltersMap,
     sFontSpecificationMap,
     sGradientStopMap,

--- a/dom/svg/SVGForeignObjectElement.h
+++ b/dom/svg/SVGForeignObjectElement.h
@@ -31,19 +31,16 @@ protected:
 public:
   // nsSVGElement specializations:
   virtual gfxMatrix PrependLocalTransformsTo(
     const gfxMatrix &aMatrix,
     SVGTransformTypes aWhich = eAllTransforms) const override;
   virtual bool HasValidDimensions() const override;
 
   // nsIContent interface
-  virtual nsresult BindToTree(nsIDocument* aDocument, nsIContent* aParent,
-                              nsIContent* aBindingParent,
-                              bool aCompileEventHandlers) override;
   NS_IMETHOD_(bool) IsAttributeMapped(const nsAtom* name) const override;
 
   virtual nsresult Clone(mozilla::dom::NodeInfo *aNodeInfo, nsINode **aResult,
                          bool aPreallocateChildren) const override;
 
   // WebIDL
   already_AddRefed<SVGAnimatedLength> X();
   already_AddRefed<SVGAnimatedLength> Y();

--- a/dom/svg/SVGSVGElement.cpp
+++ b/dom/svg/SVGSVGElement.cpp
@@ -490,21 +490,20 @@ SVGSVGElement::BindToTree(nsIDocument* a
     }
   }
 
   nsresult rv = SVGGraphicsElement::BindToTree(aDocument, aParent,
                                               aBindingParent,
                                               aCompileEventHandlers);
   NS_ENSURE_SUCCESS(rv,rv);
 
-  nsIDocument* doc = GetComposedDoc();
-  if (doc) {
-    // Setup the style sheet during binding, not element construction,
-    // because we could move the root SVG element from the document
-    // that created it to another document.
+  if (nsIDocument* doc = GetComposedDoc()) {
+    // Setup the style sheet during binding, not element construction, because
+    // we could move the root SVG element from the document that created it to
+    // another document.
     auto cache = nsLayoutStylesheetCache::Singleton();
     doc->EnsureOnDemandBuiltInUASheet(cache->SVGSheet());
   }
 
   if (mTimedDocumentRoot && smilController) {
     rv = mTimedDocumentRoot->SetParent(smilController);
     if (mStartAnimationOnBindToTree) {
       mTimedDocumentRoot->Begin();

--- a/layout/base/nsDocumentViewer.cpp
+++ b/layout/base/nsDocumentViewer.cpp
@@ -2435,29 +2435,16 @@ nsDocumentViewer::CreateStyleSet(nsIDocu
   // Make sure this does the same thing as PresShell::AddSheet wrt ordering.
 
   // this should eventually get expanded to allow for creating
   // different sets for different media
 
   UniquePtr<ServoStyleSet> styleSet = MakeUnique<ServoStyleSet>();
 
   // The document will fill in the document sheets when we create the presshell
-
-  if (aDocument->IsBeingUsedAsImage()) {
-    MOZ_ASSERT(aDocument->IsSVGDocument(),
-               "Do we want to skip most sheets for this new image type?");
-
-    // SVG-as-an-image must be kept as light and small as possible. We
-    // deliberately skip loading everything and leave svg.css (and html.css and
-    // xul.css) to be loaded on-demand.
-    // XXXjwatt Nothing else is loaded on-demand, but I don't think that
-    // should matter for SVG-as-an-image. If it does, I want to know why!
-    return styleSet;
-  }
-
   auto cache = nsLayoutStylesheetCache::Singleton();
 
   // Handle the user sheets.
   StyleSheet* sheet = nullptr;
   if (nsContentUtils::IsInChromeDocshell(aDocument)) {
     sheet = cache->UserChromeSheet();
   } else {
     sheet = cache->UserContentSheet();
@@ -2468,91 +2455,73 @@ nsDocumentViewer::CreateStyleSet(nsIDocu
   }
 
   // Append chrome sheets (scrollbars + forms).
   sheet = cache->ScrollbarsSheet();
   if (sheet) {
     styleSet->PrependStyleSheet(SheetType::Agent, sheet);
   }
 
-  if (!aDocument->IsSVGDocument()) {
-    // !!! IMPORTANT - KEEP THIS BLOCK IN SYNC WITH
-    // !!! SVGDocument::EnsureNonSVGUserAgentStyleSheetsLoaded.
-
-    // SVGForeignObjectElement::BindToTree calls SVGDocument::
-    // EnsureNonSVGUserAgentStyleSheetsLoaded to loads these UA sheet
-    // on-demand. (Excluding the quirks sheet, which should never be loaded for
-    // an SVG document, and excluding xul.css which will be loaded on demand by
-    // nsXULElement::BindToTree.)
-
-    sheet = cache->FormsSheet();
+  sheet = cache->FormsSheet();
+  if (sheet) {
+    styleSet->PrependStyleSheet(SheetType::Agent, sheet);
+  }
+
+  if (aDocument->LoadsFullXULStyleSheetUpFront()) {
+    // This is the only place components.css gets loaded, unlike xul.css
+    sheet = cache->XULComponentsSheet();
+    if (sheet) {
+      styleSet->PrependStyleSheet(SheetType::Agent, sheet);
+    }
+
+    // nsXULElement::BindToTree loads xul.css on-demand if we don't load it
+    // up-front here.
+    sheet = cache->XULSheet();
     if (sheet) {
       styleSet->PrependStyleSheet(SheetType::Agent, sheet);
     }
-
-    if (aDocument->LoadsFullXULStyleSheetUpFront()) {
-      // This is the only place components.css gets loaded, unlike xul.css
-      sheet = cache->XULComponentsSheet();
-      if (sheet) {
-        styleSet->PrependStyleSheet(SheetType::Agent, sheet);
-      }
-
-      // nsXULElement::BindToTree loads xul.css on-demand if we don't load it
-      // up-front here.
-      sheet = cache->XULSheet();
-      if (sheet) {
-        styleSet->PrependStyleSheet(SheetType::Agent, sheet);
-      }
-    }
-
-    sheet = cache->MinimalXULSheet();
-    if (sheet) {
-      // Load the minimal XUL rules for scrollbars and a few other XUL things
-      // that non-XUL (typically HTML) documents commonly use.
-      styleSet->PrependStyleSheet(SheetType::Agent, sheet);
-    }
-
-    sheet = cache->CounterStylesSheet();
+  }
+
+  sheet = cache->MinimalXULSheet();
+  if (sheet) {
+    // Load the minimal XUL rules for scrollbars and a few other XUL things
+    // that non-XUL (typically HTML) documents commonly use.
+    styleSet->PrependStyleSheet(SheetType::Agent, sheet);
+  }
+
+  sheet = cache->CounterStylesSheet();
+  if (sheet) {
+    styleSet->PrependStyleSheet(SheetType::Agent, sheet);
+  }
+
+  if (nsLayoutUtils::ShouldUseNoScriptSheet(aDocument)) {
+    sheet = cache->NoScriptSheet();
     if (sheet) {
       styleSet->PrependStyleSheet(SheetType::Agent, sheet);
     }
-
-    if (nsLayoutUtils::ShouldUseNoScriptSheet(aDocument)) {
-      sheet = cache->NoScriptSheet();
-      if (sheet) {
-        styleSet->PrependStyleSheet(SheetType::Agent, sheet);
-      }
-    }
-
-    if (nsLayoutUtils::ShouldUseNoFramesSheet(aDocument)) {
-      sheet = cache->NoFramesSheet();
-      if (sheet) {
-        styleSet->PrependStyleSheet(SheetType::Agent, sheet);
-      }
-    }
-
-    // We don't add quirk.css here; nsPresContext::CompatibilityModeChanged will
-    // append it if needed.
-
-    sheet = cache->HTMLSheet();
+  }
+
+  if (nsLayoutUtils::ShouldUseNoFramesSheet(aDocument)) {
+    sheet = cache->NoFramesSheet();
     if (sheet) {
       styleSet->PrependStyleSheet(SheetType::Agent, sheet);
     }
-
-    styleSet->PrependStyleSheet(SheetType::Agent, cache->UASheet());
-  } else {
-    // SVG documents may have scrollbars and need the scrollbar styling.
-    sheet = cache->MinimalXULSheet();
-    if (sheet) {
-      styleSet->PrependStyleSheet(SheetType::Agent, sheet);
-    }
-  }
-
-  nsStyleSheetService* sheetService = nsStyleSheetService::GetInstance();
-  if (sheetService) {
+  }
+
+  // We don't add quirk.css here; nsPresContext::CompatibilityModeChanged will
+  // append it if needed.
+
+  sheet = cache->HTMLSheet();
+  if (sheet) {
+    styleSet->PrependStyleSheet(SheetType::Agent, sheet);
+  }
+
+  styleSet->PrependStyleSheet(SheetType::Agent, cache->UASheet());
+
+  if (nsStyleSheetService* sheetService = nsStyleSheetService::GetInstance()) {
     for (StyleSheet* sheet : *sheetService->AgentStyleSheets()) {
       styleSet->AppendStyleSheet(SheetType::Agent, sheet);
     }
     for (StyleSheet* sheet : Reversed(*sheetService->UserStyleSheets())) {
       styleSet->PrependStyleSheet(SheetType::User, sheet);
     }
   }
 

--- a/layout/printing/nsPrintJob.cpp
+++ b/layout/printing/nsPrintJob.cpp
@@ -2339,23 +2339,16 @@ nsPrintJob::ReflowPrintObject(const Uniq
   aPO->mViewManager = new nsViewManager();
 
   rv = aPO->mViewManager->Init(printData->mPrintDC);
   NS_ENSURE_SUCCESS(rv,rv);
 
   UniquePtr<ServoStyleSet> styleSet =
     mDocViewerPrint->CreateStyleSet(aPO->mDocument);
 
-  if (aPO->mDocument->IsSVGDocument()) {
-    // The SVG document only loads minimal-xul.css, so it doesn't apply other
-    // styles. We should add ua.css for applying style which related to print.
-    auto cache = nsLayoutStylesheetCache::Singleton();
-    styleSet->PrependStyleSheet(SheetType::Agent, cache->UASheet());
-  }
-
   aPO->mPresShell = aPO->mDocument->CreateShell(aPO->mPresContext,
                                                 aPO->mViewManager,
                                                 std::move(styleSet));
   if (!aPO->mPresShell) {
     return NS_ERROR_FAILURE;
   }
 
   // If we're printing selection then remove the unselected nodes from our

--- a/layout/svg/svg.css
+++ b/layout/svg/svg.css
@@ -79,32 +79,8 @@ foreignObject {
   mask: inherit;
   opacity: inherit;
 }
 
 *:-moz-focusring {
   /* Don't specify the outline-color, we should always use initial value. */
   outline: 1px dotted;
 }
-
-/* nsDocumentViewer::CreateStyleSet doesn't load ua.css.
- * A few styles are common to html and SVG though
- * so we copy the rules below from that file.
- */
-
-/* Links */
-
-*|*:any-link {
-  cursor: pointer;
-}
-
-*|*:any-link:-moz-focusring {
-  /* Don't specify the outline-color, we should always use initial value. */
-  outline: 1px dotted;
-}
-
-/*
- * SVG-as-an-image needs this rule
- */
-*|*::-moz-viewport, *|*::-moz-viewport-scroll, *|*::-moz-canvas, *|*::-moz-scrolled-canvas {
-  display: block !important;
-  background-color: inherit;
-}