diff --git a/core/audits/non-composited-animations.js b/core/audits/non-composited-animations.js
index 8c6e7fb48547..e0a807b9f3ef 100644
--- a/core/audits/non-composited-animations.js
+++ b/core/audits/non-composited-animations.js
@@ -32,6 +32,14 @@ const UIStrings = {
     =1 {Unsupported CSS Property: {properties}}
     other {Unsupported CSS Properties: {properties}}
   }`,
+  /**
+   * @description [ICU Syntax] Descriptive reason for why a user-provided animation failed to be optimized by the browser due to custom CSS properties (CSS variables) not being supported on the compositor. Shown in a table with a list of other potential failure reasons.
+   * @example {--swing-y, --rotation} properties
+   */
+  unsupportedCustomCSSProperty: `{propertyCount, plural,
+    =1 {Custom CSS properties cannot be animated on the compositor: {properties}}
+    other {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.  */
@@ -90,14 +98,44 @@ function getActionableFailureReasons(failureCode, unsupportedProperties) {
   return ACTIONABLE_FAILURE_REASONS
     .filter(reason => failureCode & reason.flag)
     .map(reason => {
+      // Handle both regular CSS properties and custom CSS properties
       if (reason.text === UIStrings.unsupportedCSSProperty) {
-        return str_(reason.text, {
-          propertyCount: unsupportedProperties.length,
-          properties: unsupportedProperties.join(', '),
-        });
+        const customProperties = new Set();
+        const nonCustomProperties = new Set();
+
+        // Separate custom properties (starting with '--') from regular properties
+        for (const property of unsupportedProperties) {
+          if (property.startsWith('--')) {
+            customProperties.add(property);
+          } else {
+            nonCustomProperties.add(property);
+          }
+        }
+
+        const reasons = [];
+
+        // Add regular CSS properties message if any exist
+        if (nonCustomProperties.size > 0) {
+          reasons.push(str_(UIStrings.unsupportedCSSProperty, {
+            propertyCount: nonCustomProperties.size,
+            properties: Array.from(nonCustomProperties).join(', '),
+          }));
+        }
+
+        // Add custom CSS properties message if any exist
+        if (customProperties.size > 0) {
+          reasons.push(str_(UIStrings.unsupportedCustomCSSProperty, {
+            propertyCount: customProperties.size,
+            properties: Array.from(customProperties).join(', '),
+          }));
+        }
+
+        return reasons;
       }
+
       return str_(reason.text);
-    });
+    })
+    .flat(); // Flatten array since we might return multiple messages for unsupported properties
 }
 
 class NonCompositedAnimations extends Audit {
diff --git a/core/test/audits/non-composited-animations-test.js b/core/test/audits/non-composited-animations-test.js
index 11f99eb02479..154bee29ce1c 100644
--- a/core/test/audits/non-composited-animations-test.js
+++ b/core/test/audits/non-composited-animations-test.js
@@ -233,4 +233,129 @@ describe('Non-composited animations audit', () => {
     expect(auditResult.details.items[0].subItems.items[5].animation)
       .toBeUndefined();
   });
+
+  // Testing custom CSS property separation
+  it('separates custom CSS properties from regular properties', async () => {
+    const artifacts = {
+      TraceElements: [
+        {
+          traceEventType: 'animation',
+          nodeId: 4,
+          node: {
+            devtoolsNodePath: '1,HTML,1,BODY,1,DIV',
+            selector: 'body > div#custom-animated',
+            nodeLabel: 'div',
+            snippet: '<div id="custom-animated">',
+          },
+          animations: [
+            {
+              name: 'customAnimation',
+              failureReasonsMask: 8192,
+              unsupportedProperties: ['--swing-y', '--rotation', 'color', 'height'],
+            },
+          ],
+        },
+      ],
+      HostUserAgent: 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_6) ' +
+        'AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4216.0 Safari/537.36',
+    };
+
+    const auditResult = await NonCompositedAnimationsAudit.audit(artifacts);
+    expect(auditResult.score).toEqual(0);
+    expect(auditResult.details.headings).toHaveLength(2);
+    expect(auditResult.displayValue).toBeDisplayString('1 animated element found');
+    expect(auditResult.details.items).toHaveLength(1);
+
+    const subItems = auditResult.details.items[0].subItems.items;
+
+    // There should be two separate messages: one for standard CSS properties, and one for custom properties.
+    expect(subItems).toHaveLength(2);
+
+    const failureReasons = subItems.map(item => item.failureReason);
+
+    expect(failureReasons[0])
+      .toBeDisplayString('Unsupported CSS Properties: color, height');
+    expect(failureReasons[1])
+      .toBeDisplayString(
+        'Custom CSS properties cannot be animated on the compositor: --swing-y, --rotation'
+      );
+
+    expect(subItems[0].animation).toEqual('customAnimation');
+    expect(subItems[1].animation).toEqual('customAnimation');
+  });
+
+  // Custom properties only
+  it('handles animations with only custom CSS properties', async () => {
+    const artifacts = {
+      TraceElements: [
+        {
+          traceEventType: 'animation',
+          nodeId: 5,
+          node: {
+            devtoolsNodePath: '1,HTML,1,BODY,1,DIV',
+            selector: 'body > div#only-custom',
+            nodeLabel: 'div',
+            snippet: '<div id="only-custom">',
+          },
+          animations: [
+            {
+              failureReasonsMask: 8192,
+              unsupportedProperties: ['--yheight'],
+            },
+          ],
+        },
+      ],
+      HostUserAgent: 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_6) ' +
+        'AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4216.0 Safari/537.36',
+    };
+
+    const auditResult = await NonCompositedAnimationsAudit.audit(artifacts);
+    expect(auditResult.score).toEqual(0);
+    expect(auditResult.details.items).toHaveLength(1);
+
+    const subItems = auditResult.details.items[0].subItems.items;
+    expect(subItems).toHaveLength(1);
+    expect(subItems[0].failureReason)
+      .toBeDisplayString(
+        'Custom CSS properties cannot be animated on the compositor: --yheight'
+      );
+    expect(subItems[0].animation).toBeUndefined();
+  });
+
+  // In the case of general properties only
+  it('handles animations with only regular CSS properties', async () => {
+    const artifacts = {
+      TraceElements: [
+        {
+          traceEventType: 'animation',
+          nodeId: 6,
+          node: {
+            devtoolsNodePath: '1,HTML,1,BODY,1,DIV',
+            selector: 'body > div#only-regular',
+            nodeLabel: 'div',
+            snippet: '<div id="only-regular">',
+          },
+          animations: [
+            {
+              name: 'regularAnimation',
+              failureReasonsMask: 8192,
+              unsupportedProperties: ['margin', 'padding'],
+            },
+          ],
+        },
+      ],
+      HostUserAgent: 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_6) ' +
+        'AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4216.0 Safari/537.36',
+    };
+
+    const auditResult = await NonCompositedAnimationsAudit.audit(artifacts);
+    expect(auditResult.score).toEqual(0);
+    expect(auditResult.details.items).toHaveLength(1);
+
+    const subItems = auditResult.details.items[0].subItems.items;
+    expect(subItems).toHaveLength(1);
+    expect(subItems[0].failureReason)
+      .toBeDisplayString('Unsupported CSS Properties: margin, padding');
+    expect(subItems[0].animation).toEqual('regularAnimation');
+  });
 });
diff --git a/shared/localization/locales/en-US.json b/shared/localization/locales/en-US.json
index 0abe2fa96034..bdcfd83f69ef 100644
--- a/shared/localization/locales/en-US.json
+++ b/shared/localization/locales/en-US.json
@@ -1229,6 +1229,9 @@
   "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": "{propertyCount, plural,\n    =1 {Custom CSS properties cannot be animated on the compositor: {properties}}\n    other {Custom CSS properties cannot be animated on the compositor: {properties}}\n  }"
+  },
   "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 0e4d595438bf..66344d16a7b0 100644
--- a/shared/localization/locales/en-XL.json
+++ b/shared/localization/locales/en-XL.json
@@ -1229,6 +1229,9 @@
   "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": "{propertyCount, plural,\n    =1 {Ĉúŝt́ôḿ ĈŚŜ ṕr̂óp̂ér̂t́îéŝ ćâńn̂ót̂ b́ê án̂ím̂át̂éd̂ ón̂ t́ĥé ĉóm̂ṕôśît́ôŕ: {properties}}\n    other {Ĉúŝt́ôḿ ĈŚŜ ṕr̂óp̂ér̂t́îéŝ ćâńn̂ót̂ b́ê án̂ím̂át̂éd̂ ón̂ t́ĥé ĉóm̂ṕôśît́ôŕ: {properties}}\n  }"
+  },
   "core/audits/non-composited-animations.js | unsupportedTimingParameters": {
     "message": "Êf́f̂éĉt́ ĥáŝ ún̂śûṕp̂ór̂t́êd́ t̂ím̂ín̂ǵ p̂ár̂ám̂ét̂ér̂ś"
   },