diff --git a/core/audits/non-composited-animations.js b/core/audits/non-composited-animations.js
index 8c6e7fb48547..434d175764b1 100644
--- a/core/audits/non-composited-animations.js
+++ b/core/audits/non-composited-animations.js
@@ -32,6 +32,12 @@ const UIStrings = {
     =1 {Unsupported CSS Property: {properties}}
     other {Unsupported CSS Properties: {properties}}
   }`,
+  /**
+   * @description Descriptive reason for why a user-provided animation failed to be optimized by the browser due to the animated custom CSS property not being supported on the compositor. Shown in a table with a list of other potential failure reasons.
+   * @example {--my-custom-property} properties
+   * */
+  unsupportedCustomCSSProperty: 'Custom CSS properties cannot be animated on the compositor: ' +
+  '{properties}',
   /** Descriptive reason for why a user-provided animation failed to be optimized by the browser due to a `transform` property being dependent on the size of the element itself. Shown in a table with a list of other potential failure reasons.  */
   transformDependsBoxSize: 'Transform-related property depends on box size',
   /** Descriptive reason for why a user-provided animation failed to be optimized by the browser due to a `filter` property possibly moving pixels. Shown in a table with a list of other potential failure reasons.  */
@@ -56,6 +62,10 @@ const ACTIONABLE_FAILURE_REASONS = [
     flag: 1 << 13,
     text: UIStrings.unsupportedCSSProperty,
   },
+  {
+    flag: 1 << 13,
+    text: UIStrings.unsupportedCustomCSSProperty,
+  },
   {
     flag: 1 << 11,
     text: UIStrings.transformDependsBoxSize,
@@ -91,9 +101,18 @@ function getActionableFailureReasons(failureCode, unsupportedProperties) {
     .filter(reason => failureCode & reason.flag)
     .map(reason => {
       if (reason.text === UIStrings.unsupportedCSSProperty) {
+        const nonCustomUnSupportedProperties = unsupportedProperties
+        .filter(property => !property.startsWith('--'));
         return str_(reason.text, {
-          propertyCount: unsupportedProperties.length,
-          properties: unsupportedProperties.join(', '),
+          propertyCount: nonCustomUnSupportedProperties.length,
+          properties: nonCustomUnSupportedProperties.join(', '),
+        });
+      }
+      if (reason.text === UIStrings.unsupportedCustomCSSProperty) {
+        const customUnsupportedProperties = unsupportedProperties
+        .filter(property => property.startsWith('--'));
+        return str_(UIStrings.unsupportedCustomCSSProperty, {
+          properties: customUnsupportedProperties.join(', '),
         });
       }
       return str_(reason.text);
diff --git a/shared/localization/locales/en-US.json b/shared/localization/locales/en-US.json
index 005a526db204..a4cfeb22854e 100644
--- a/shared/localization/locales/en-US.json
+++ b/shared/localization/locales/en-US.json
@@ -1292,6 +1292,9 @@
   "core/audits/non-composited-animations.js | unsupportedCSSProperty": {
     "message": "{propertyCount, plural,\n    =1 {Unsupported CSS Property: {properties}}\n    other {Unsupported CSS Properties: {properties}}\n  }"
   },
+  "core/audits/non-composited-animations.js | unsupportedCustomCSSProperty": {
+    "message": "Custom CSS properties cannot be animated on the compositor: {properties}"
+  },
   "core/audits/non-composited-animations.js | unsupportedTimingParameters": {
     "message": "Effect has unsupported timing parameters"
   },
diff --git a/shared/localization/locales/en-XL.json b/shared/localization/locales/en-XL.json
index 3f1cf397915f..4ca991807dfd 100644
--- a/shared/localization/locales/en-XL.json
+++ b/shared/localization/locales/en-XL.json
@@ -1292,6 +1292,9 @@
   "core/audits/non-composited-animations.js | unsupportedCSSProperty": {
     "message": "{propertyCount, plural,\n    =1 {Ûńŝúp̂ṕôŕt̂éd̂ ĆŜŚ P̂ŕôṕêŕt̂ý: {properties}}\n    other {Ûńŝúp̂ṕôŕt̂éd̂ ĆŜŚ P̂ŕôṕêŕt̂íêś: {properties}}\n  }"
   },
+  "core/audits/non-composited-animations.js | unsupportedCustomCSSProperty": {
+    "message": "Ĉúŝt́ôḿ ĈŚŜ ṕr̂óp̂ér̂t́îéŝ ćâńn̂ót̂ b́ê án̂ím̂át̂éd̂ ón̂ t́ĥé ĉóm̂ṕôśît́ôŕ: {properties}"
+  },
   "core/audits/non-composited-animations.js | unsupportedTimingParameters": {
     "message": "Êf́f̂éĉt́ ĥáŝ ún̂śûṕp̂ór̂t́êd́ t̂ím̂ín̂ǵ p̂ár̂ám̂ét̂ér̂ś"
   },