diff --git a/lib/checks/aria/aria-valid-attr-value-evaluate.js b/lib/checks/aria/aria-valid-attr-value-evaluate.js
index 469f29186f..0b6007ff42 100644
--- a/lib/checks/aria/aria-valid-attr-value-evaluate.js
+++ b/lib/checks/aria/aria-valid-attr-value-evaluate.js
@@ -67,7 +67,9 @@ function ariaValidAttrValueEvaluate(node, options, virtualNode) {
         needsReview = `aria-describedby="${virtualNode.attr(
           'aria-describedby'
         )}"`;
-        messageKey = 'noId';
+        // TODO: es-modules_tree
+        messageKey =
+          axe._tree && axe._tree[0]._hasShadowRoot ? 'noIdShadow' : 'noId';
       }
 
       return;
@@ -80,7 +82,9 @@ function ariaValidAttrValueEvaluate(node, options, virtualNode) {
         needsReview = `aria-labelledby="${virtualNode.attr(
           'aria-labelledby'
         )}"`;
-        messageKey = 'noId';
+        // TODO: es-modules_tree
+        messageKey =
+          axe._tree && axe._tree[0]._hasShadowRoot ? 'noIdShadow' : 'noId';
       }
     }
   };
diff --git a/lib/checks/aria/aria-valid-attr-value.json b/lib/checks/aria/aria-valid-attr-value.json
index 7c14f3d59f..12658c4149 100644
--- a/lib/checks/aria/aria-valid-attr-value.json
+++ b/lib/checks/aria/aria-valid-attr-value.json
@@ -12,6 +12,7 @@
       },
       "incomplete": {
         "noId": "ARIA attribute element ID does not exist on the page: ${data.needsReview}",
+        "noIdShadow": "ARIA attribute element ID does not exist on the page or is a descendant of a different shadow DOM tree: ${data.needsReview}",
         "ariaCurrent": "ARIA attribute value is invalid and will be treated as \"aria-current=true\": ${data.needsReview}",
         "idrefs": "Unable to determine if ARIA attribute element ID exists on the page: ${data.needsReview}"
       }
diff --git a/lib/core/utils/get-flattened-tree.js b/lib/core/utils/get-flattened-tree.js
index af50400157..6bddc10ab3 100644
--- a/lib/core/utils/get-flattened-tree.js
+++ b/lib/core/utils/get-flattened-tree.js
@@ -20,6 +20,8 @@ import cache from '../base/cache';
  *    the spec for this)
  */
 
+let hasShadowRoot;
+
 /**
  * find all the fallback content for a <slot> and return these as an array
  * this array will also include any #text nodes
@@ -65,6 +67,8 @@ function flattenTree(node, shadowId, parent) {
   nodeName = node.nodeName.toLowerCase();
 
   if (isShadowRoot(node)) {
+    hasShadowRoot = true;
+
     // generate an ID for this shadow root and overwrite the current
     // closure shadowId with this value so that it cascades down the tree
     retVal = new VirtualNode(node, parent, shadowId);
@@ -140,12 +144,19 @@ function flattenTree(node, shadowId, parent) {
  *                           ancestor of the node
  */
 function getFlattenedTree(node = document.documentElement, shadowId) {
+  hasShadowRoot = false;
   cache.set('nodeMap', new WeakMap());
 
   // specifically pass `null` to the parent to designate the top
   // node of the tree. if parent === undefined then we know
   // we are in a disconnected tree
-  return flattenTree(node, shadowId, null);
+  const tree = flattenTree(node, shadowId, null);
+
+  // allow rules and checks to know if there is a shadow root attached
+  // to the current tree
+  tree[0]._hasShadowRoot = hasShadowRoot;
+
+  return tree;
 }
 
 export default getFlattenedTree;
diff --git a/test/checks/aria/valid-attr-value.js b/test/checks/aria/valid-attr-value.js
index 41a49b9e1f..8ad93a95eb 100644
--- a/test/checks/aria/valid-attr-value.js
+++ b/test/checks/aria/valid-attr-value.js
@@ -5,6 +5,8 @@ describe('aria-valid-attr-value', function() {
   var queryFixture = axe.testUtils.queryFixture;
   var checkContext = axe.testUtils.MockCheckContext();
   var fixtureSetup = axe.testUtils.fixtureSetup;
+  var shadowCheckSetup = axe.testUtils.shadowCheckSetup;
+  var shadowSupported = axe.testUtils.shadowSupport.v1;
 
   afterEach(function() {
     fixture.innerHTML = '';
@@ -210,8 +212,31 @@ describe('aria-valid-attr-value', function() {
         .getCheckEvaluate('aria-valid-attr-value')
         .call(checkContext, null, null, vNode)
     );
+    assert.deepEqual(checkContext._data, {
+      messageKey: 'noId',
+      needsReview: 'aria-describedby="test"'
+    });
   });
 
+  (shadowSupported ? it : xit)(
+    'should return undefined on aria-describedby when the element is in a different shadow tree',
+    function() {
+      var params = shadowCheckSetup(
+        '<div></div>',
+        '<button id="target" aria-describedby="test">Button</button>'
+      );
+      assert.isUndefined(
+        axe.testUtils
+          .getCheckEvaluate('aria-valid-attr-value')
+          .apply(checkContext, params)
+      );
+      assert.deepEqual(checkContext._data, {
+        messageKey: 'noIdShadow',
+        needsReview: 'aria-describedby="test"'
+      });
+    }
+  );
+
   it('should return undefined on aria-labelledby when the element is not in the DOM', function() {
     var vNode = queryFixture(
       '<button id="target" aria-labelledby="test">Button</button>'
@@ -221,8 +246,31 @@ describe('aria-valid-attr-value', function() {
         .getCheckEvaluate('aria-valid-attr-value')
         .call(checkContext, null, null, vNode)
     );
+    assert.deepEqual(checkContext._data, {
+      messageKey: 'noId',
+      needsReview: 'aria-labelledby="test"'
+    });
   });
 
+  (shadowSupported ? it : xit)(
+    'should return undefined on aria-labelledby when the element is in a different shadow tree',
+    function() {
+      var params = shadowCheckSetup(
+        '<div></div>',
+        '<button id="target" aria-labelledby="test">Button</button>'
+      );
+      assert.isUndefined(
+        axe.testUtils
+          .getCheckEvaluate('aria-valid-attr-value')
+          .apply(checkContext, params)
+      );
+      assert.deepEqual(checkContext._data, {
+        messageKey: 'noIdShadow',
+        needsReview: 'aria-labelledby="test"'
+      });
+    }
+  );
+
   it('should return undefined on aria-current with invalid value', function() {
     var vNode = queryFixture(
       '<button id="target" aria-current="test">Button</button>'