diff --git a/layout/inspector/inCSSValueSearch.cpp b/layout/inspector/inCSSValueSearch.cpp
index 5897e1b542..82697508ed 100644
--- a/layout/inspector/inCSSValueSearch.cpp
+++ b/layout/inspector/inCSSValueSearch.cpp
@@ -18,6 +18,8 @@
 #include "nsIDOMCSSImportRule.h"
 #include "nsIDOMCSSMediaRule.h"
 #include "nsIDOMCSSSupportsRule.h"
+#include "nsIDOMCSSLayerBlockRule.h"
+#include "nsIDOMCSSLayerStatementRule.h"
 #include "nsIDOMCSSRule.h"
 #include "nsIURI.h"
 #include "nsIDocument.h"
@@ -322,6 +324,12 @@ inCSSValueSearch::SearchRuleList(nsIDOMCSSRuleList* aRuleList, nsIURI* aBaseURL)
         supportsRule->GetCssRules(getter_AddRefs(childRules));
         SearchRuleList(childRules, aBaseURL);
       } break;
+      case nsIDOMCSSRule::LAYER_BLOCK_RULE: {
+        nsCOMPtr<nsIDOMCSSLayerBlockRule> layerBlockRule = do_QueryInterface(rule);
+        nsCOMPtr<nsIDOMCSSRuleList> childRules;
+        layerBlockRule->GetCssRules(getter_AddRefs(childRules));
+        SearchRuleList(childRules, aBaseURL);
+      } break;
       default:
         // XXX handle nsIDOMCSSRule::PAGE_RULE if we ever support it
         break;
diff --git a/layout/style/CSSStyleSheet.cpp b/layout/style/CSSStyleSheet.cpp
index 1307864c9d..1ed14259db 100644
--- a/layout/style/CSSStyleSheet.cpp
+++ b/layout/style/CSSStyleSheet.cpp
@@ -1896,6 +1896,8 @@ CSSStyleSheet::InsertRuleIntoGroup(const nsAString & aRule,
     case css::Rule::COUNTER_STYLE_RULE:
     case css::Rule::DOCUMENT_RULE:
     case css::Rule::SUPPORTS_RULE:
+    case css::Rule::LAYER_BLOCK_RULE:
+    case css::Rule::LAYER_STATEMENT_RULE:
       // these types are OK to insert into a group
       break;
     case css::Rule::CHARSET_RULE:
diff --git a/layout/style/nsCSSParser.cpp b/layout/style/nsCSSParser.cpp
index 72b105ca3f..1603e66353 100644
--- a/layout/style/nsCSSParser.cpp
+++ b/layout/style/nsCSSParser.cpp
@@ -68,6 +68,7 @@ static bool sWebkitDevicePixelRatioEnabled;
 static bool sMozGradientsEnabled;
 static bool sControlCharVisibility;
 static bool sLegacyNegationPseudoClassEnabled;
+static bool sCascadeLayersEnabled;
 
 const uint32_t
 nsCSSProps::kParserVariantTable[eCSSProperty_COUNT_no_shorthands] = {
@@ -753,6 +754,8 @@ protected:
                                    nsCSSValue& aValue);
   bool ParseCounterRange(nsCSSValuePair& aPair);
 
+  bool ParseLayerRule(RuleAppendFunc aAppendFunc, void* aProcessData);
+
   /**
    * Parses the current input stream for a CSS token stream value and resolves
    * any variable references using the variables in aVariables.
@@ -3453,6 +3456,11 @@ CSSParserImpl::ParseAtRule(RuleAppendFunc aAppendFunc,
     parseFunc = &CSSParserImpl::ParseSupportsRule;
     newSection = eCSSSection_General;
 
+  } else if (mToken.mIdent.LowerCaseEqualsLiteral("layer") &&
+             sCascadeLayersEnabled) {
+    parseFunc = &CSSParserImpl::ParseLayerRule;
+    newSection = eCSSSection_General;
+
   } else if (mToken.mIdent.LowerCaseEqualsLiteral("counter-style")) {
     parseFunc = &CSSParserImpl::ParseCounterStyleRule;
     newSection = eCSSSection_General;
@@ -4935,6 +4943,82 @@ CSSParserImpl::ParseSupportsConditionTermsAfterOperator(
   }
 }
 
+bool
+CSSParserImpl::ParseLayerRule(RuleAppendFunc aAppendFunc, void* aProcessData)
+{
+  nsString layerName;
+
+  uint32_t linenum, colnum;
+  if (!GetNextTokenLocation(true, &linenum, &colnum)) {
+    return false;
+  }
+
+  nsCSSToken* tk = &mToken;
+  if (!GetToken(true)) {
+    return false;
+  }
+
+  // Parse the layer name or name list if we aren't immediately
+  // followed by a "{", which indicates an anonymous layer.
+  if (tk->mType == eCSSToken_Ident) {
+    nsTArray<nsString>* nameList = new nsTArray<nsString>();
+    nameList->AppendElement(tk->mIdent);
+
+    bool parsing = true;
+    bool expectIdent = false;
+    while (parsing) {
+      if (!GetToken(true)) {
+        return false;
+      }
+
+      switch (tk->mType) {
+        case eCSSToken_Symbol: {
+          if (',' == tk->mSymbol) {
+            if (expectIdent) {
+              return false;
+            }
+            expectIdent = true;
+            continue;
+          } else if (';' == tk->mSymbol) {
+            if (expectIdent) {
+              return false;
+            }
+            RefPtr<CSSLayerStatementRule> rule =
+              new CSSLayerStatementRule(*nameList, linenum, colnum);
+            return true;
+          } else if ('{' == tk->mSymbol) {
+            if (expectIdent) {
+              return false;
+            }
+            uint32_t nameListLength = nameList->Length();
+            if (nameListLength == 0 || nameListLength > 1) {
+              return false;
+            }
+            layerName.Assign(nameList->ElementAt(0));
+            UngetToken();
+            parsing = false;
+            break;
+          }
+        }
+        case eCSSToken_Ident: {
+          nameList->AppendElement(tk->mIdent);
+          expectIdent = false;
+          break;
+        }
+        default: {
+          return false;
+        }
+      }
+    }
+  } else if (tk->mType == eCSSToken_Symbol && '{' != tk->mSymbol) {
+    return false;
+  }
+
+  RefPtr<css::GroupRule> rule =
+    new CSSLayerBlockRule(layerName, linenum, colnum);
+  return ParseGroupRule(rule, aAppendFunc, aProcessData);
+}
+
 bool
 CSSParserImpl::ParseCounterStyleRule(RuleAppendFunc aAppendFunc, void* aData)
 {
@@ -18185,6 +18269,8 @@ nsCSSParser::Startup()
                                "layout.css.control-characters.visible");
   Preferences::AddBoolVarCache(&sLegacyNegationPseudoClassEnabled,
                                "layout.css.legacy-negation-pseudo.enabled");
+  Preferences::AddBoolVarCache(&sCascadeLayersEnabled,
+                               "layout.css.cascade-layers.enabled");
 }
 
 nsCSSParser::nsCSSParser(mozilla::css::Loader* aLoader,
diff --git a/layout/style/nsCSSRuleProcessor.cpp b/layout/style/nsCSSRuleProcessor.cpp
index ff3661c73d..0a9c55fc17 100644
--- a/layout/style/nsCSSRuleProcessor.cpp
+++ b/layout/style/nsCSSRuleProcessor.cpp
@@ -3932,7 +3932,8 @@ GatherDocRuleEnumFunc(css::Rule* aRule, void* aData)
              "mMustGatherDocumentRules is true");
 
   if (css::Rule::MEDIA_RULE == type ||
-      css::Rule::SUPPORTS_RULE == type) {
+      css::Rule::SUPPORTS_RULE == type ||
+      css::Rule::LAYER_BLOCK_RULE == type) {
     css::GroupRule* groupRule = static_cast<css::GroupRule*>(aRule);
     if (!groupRule->EnumerateRulesForwards(GatherDocRuleEnumFunc, aData)) {
       return false;
@@ -4011,7 +4012,8 @@ CascadeRuleEnumFunc(css::Rule* aRule, void* aData)
     }
   }
   else if (css::Rule::MEDIA_RULE == type ||
-           css::Rule::SUPPORTS_RULE == type) {
+           css::Rule::SUPPORTS_RULE == type ||
+           css::Rule::LAYER_BLOCK_RULE == type) {
     css::GroupRule* groupRule = static_cast<css::GroupRule*>(aRule);
     const bool use =
       groupRule->UseForPresentation(data->mPresContext, data->mCacheKey);