diff --git a/accessible/windows/sdn/sdnAccessible.cpp b/accessible/windows/sdn/sdnAccessible.cpp
index b248edbbfab4..6c6089a1cd12 100644
--- a/accessible/windows/sdn/sdnAccessible.cpp
+++ b/accessible/windows/sdn/sdnAccessible.cpp
@@ -229,8 +229,7 @@ sdnAccessible::get_computedStyle(unsigned short aMaxStyleProperties,
     nsWinUtils::GetComputedStyleDeclaration(mNode->AsContent());
   NS_ENSURE_TRUE(cssDecl, E_FAIL);
 
-  uint32_t length = 0;
-  cssDecl->GetLength(&length);
+  uint32_t length = cssDecl->Length();
 
   uint32_t index = 0, realIndex = 0;
   for (index = realIndex = 0; index < length && realIndex < aMaxStyleProperties;
diff --git a/editor/libeditor/CSSEditUtils.cpp b/editor/libeditor/CSSEditUtils.cpp
index 4e57f497cbab..a8331570e6c1 100644
--- a/editor/libeditor/CSSEditUtils.cpp
+++ b/editor/libeditor/CSSEditUtils.cpp
@@ -1325,7 +1325,7 @@ CSSEditUtils::GetInlineStyles(Element* aElement,
   MOZ_ASSERT(cssDecl);
 
   cssDecl.forget(aCssDecl);
-  (*aCssDecl)->GetLength(aLength);
+  *aLength = (*aCssDecl)->Length();
   return NS_OK;
 }
 
diff --git a/editor/libeditor/ChangeStyleTransaction.cpp b/editor/libeditor/ChangeStyleTransaction.cpp
index 547245d62053..0896405cd7fa 100644
--- a/editor/libeditor/ChangeStyleTransaction.cpp
+++ b/editor/libeditor/ChangeStyleTransaction.cpp
@@ -220,9 +220,7 @@ ChangeStyleTransaction::DoTransaction()
   }
 
   // Let's be sure we don't keep an empty style attribute
-  uint32_t length;
-  rv = cssDecl->GetLength(&length);
-  NS_ENSURE_SUCCESS(rv, rv);
+  uint32_t length = cssDecl->Length();
   if (!length) {
     rv = mElement->UnsetAttr(kNameSpaceID_None, nsGkAtoms::style, true);
     NS_ENSURE_SUCCESS(rv, rv);
diff --git a/layout/style/nsCSSFontFaceRule.cpp b/layout/style/nsCSSFontFaceRule.cpp
index 8ca0eb6421ef..9e7ffd47326a 100644
--- a/layout/style/nsCSSFontFaceRule.cpp
+++ b/layout/style/nsCSSFontFaceRule.cpp
@@ -225,18 +225,19 @@ nsCSSFontFaceStyleDecl::SetProperty(const nsAString& propertyName,
   return NS_ERROR_NOT_IMPLEMENTED; // bug 443978
 }
 
-NS_IMETHODIMP
-nsCSSFontFaceStyleDecl::GetLength(uint32_t *aLength)
+uint32_t
+nsCSSFontFaceStyleDecl::Length()
 {
   uint32_t len = 0;
   for (nsCSSFontDesc id = nsCSSFontDesc(eCSSFontDesc_UNKNOWN + 1);
        id < eCSSFontDesc_COUNT;
-       id = nsCSSFontDesc(id + 1))
-    if (mDescriptors.Get(id).GetUnit() != eCSSUnit_Null)
+       id = nsCSSFontDesc(id + 1)) {
+    if (mDescriptors.Get(id).GetUnit() != eCSSUnit_Null) {
       len++;
+    }
+  }
 
-  *aLength = len;
-  return NS_OK;
+  return len;
 }
 
 void
diff --git a/layout/style/nsComputedDOMStyle.cpp b/layout/style/nsComputedDOMStyle.cpp
index 8ffca315da71..a4d0b5f3e7bd 100644
--- a/layout/style/nsComputedDOMStyle.cpp
+++ b/layout/style/nsComputedDOMStyle.cpp
@@ -439,11 +439,9 @@ nsComputedDOMStyle::SetCssText(const nsAString& aCssText,
   aRv.Throw(NS_ERROR_DOM_NO_MODIFICATION_ALLOWED_ERR);
 }
 
-NS_IMETHODIMP
-nsComputedDOMStyle::GetLength(uint32_t* aLength)
+uint32_t
+nsComputedDOMStyle::Length()
 {
-  NS_PRECONDITION(aLength, "Null aLength!  Prepare to die!");
-
   uint32_t length = GetComputedStyleMap()->Length();
 
   // Make sure we have up to date style so that we can include custom
@@ -455,11 +453,9 @@ nsComputedDOMStyle::GetLength(uint32_t* aLength)
       : StyleVariables()->mVariables.Count();
   }
 
-  *aLength = length;
-
   ClearCurrentStyleSources();
 
-  return NS_OK;
+  return length;
 }
 
 css::Rule*
diff --git a/layout/style/nsDOMCSSDeclaration.cpp b/layout/style/nsDOMCSSDeclaration.cpp
index ec9d2b86a377..a98f30c7d58c 100644
--- a/layout/style/nsDOMCSSDeclaration.cpp
+++ b/layout/style/nsDOMCSSDeclaration.cpp
@@ -161,18 +161,16 @@ nsDOMCSSDeclaration::SetCssText(const nsAString& aCssText,
   aRv = SetCSSDeclaration(newdecl);
 }
 
-NS_IMETHODIMP
-nsDOMCSSDeclaration::GetLength(uint32_t* aLength)
+uint32_t
+nsDOMCSSDeclaration::Length()
 {
   DeclarationBlock* decl = GetCSSDeclaration(eOperation_Read);
 
   if (decl) {
-    *aLength = decl->Count();
-  } else {
-    *aLength = 0;
+    return decl->Count();
   }
 
-  return NS_OK;
+  return 0;
 }
 
 already_AddRefed<dom::CSSValue>
diff --git a/layout/style/nsDOMCSSDeclaration.h b/layout/style/nsDOMCSSDeclaration.h
index 89cc4b6dfe7c..04cdf39c7e4d 100644
--- a/layout/style/nsDOMCSSDeclaration.h
+++ b/layout/style/nsDOMCSSDeclaration.h
@@ -43,8 +43,6 @@ public:
   NS_IMETHOD_(MozExternalRefCountType) AddRef() override = 0;
   NS_IMETHOD_(MozExternalRefCountType) Release() override = 0;
 
-  using nsICSSDeclaration::GetLength;
-
   /**
    * Method analogous to CSSStyleDeclaration::GetPropertyValue,
    * which obeys all the same restrictions.
@@ -81,7 +79,7 @@ public:
                          const nsAString& value,
                          const nsAString& priority,
                          nsIPrincipal* aSubjectPrincipal) override;
-  NS_IMETHOD GetLength(uint32_t *aLength) override;
+  uint32_t Length() override;
 
   // WebIDL interface for CSS2Properties
 #define CSS_PROP_PUBLIC_OR_PRIVATE(publicname_, privatename_) publicname_
diff --git a/layout/style/nsICSSDeclaration.h b/layout/style/nsICSSDeclaration.h
index aaf6de097f90..5ca4675633bc 100644
--- a/layout/style/nsICSSDeclaration.h
+++ b/layout/style/nsICSSDeclaration.h
@@ -62,7 +62,6 @@ public:
                          const nsAString& aValue,
                          const nsAString& aPriority,
                          nsIPrincipal* aSubjectPrincipal = nullptr) = 0;
-  NS_IMETHOD GetLength(uint32_t* aLength) = 0;
   void Item(uint32_t aIndex, nsAString& aReturn)
   {
     bool found;
@@ -77,11 +76,7 @@ public:
                           nsIPrincipal* aSubjectPrincipal,
                           mozilla::ErrorResult& rv) = 0;
   virtual void GetCssText(nsAString& aString) = 0;
-  uint32_t Length() {
-    uint32_t length;
-    GetLength(&length);
-    return length;
-  }
+  virtual uint32_t Length() = 0;
   void Item(uint32_t aIndex, nsString& aPropName) {
     Item(aIndex, static_cast<nsAString&>(aPropName));
   }
@@ -122,7 +117,7 @@ NS_DEFINE_STATIC_IID_ACCESSOR(nsICSSDeclaration, NS_ICSSDECLARATION_IID)
                          const nsAString& value,                              \
                          const nsAString& priority,                           \
                          nsIPrincipal* aSubjectPrincipal = nullptr) override; \
-  NS_IMETHOD GetLength(uint32_t *aLength) override; \
+  uint32_t Length() override;                                           \
   mozilla::css::Rule* GetParentRule() override;
 
 #endif // nsICSSDeclaration_h__