diff --git a/dom/base/AnonymousContent.cpp b/dom/base/AnonymousContent.cpp
index 1df36b0483..aea923f2bd 100644
--- a/dom/base/AnonymousContent.cpp
+++ b/dom/base/AnonymousContent.cpp
@@ -7,6 +7,7 @@
 #include "AnonymousContent.h"
 #include "mozilla/dom/Element.h"
 #include "mozilla/dom/AnonymousContentBinding.h"
+#include "nsComputedDOMStyle.h"
 #include "nsCycleCollectionParticipant.h"
 #include "nsIDocument.h"
 #include "nsIDOMHTMLCollection.h"
@@ -208,5 +209,31 @@ AnonymousContent::WrapObject(JSContext* aCx,
   return AnonymousContentBinding::Wrap(aCx, this, aGivenProto, aReflector);
 }
 
+void
+AnonymousContent::GetComputedStylePropertyValue(const nsAString& aElementId,
+                                                const nsAString& aPropertyName,
+                                                DOMString& aResult,
+                                                ErrorResult& aRv)
+{
+  Element* element = GetElementById(aElementId);
+  if (!element) {
+    aRv.Throw(NS_ERROR_NOT_AVAILABLE);
+    return;
+  }
+
+  nsIPresShell* shell = element->OwnerDoc()->GetShell();
+  if (!shell) {
+    aRv.Throw(NS_ERROR_NOT_AVAILABLE);
+    return;
+  }
+
+  RefPtr<nsComputedDOMStyle> cs =
+    new nsComputedDOMStyle(element,
+                           NS_LITERAL_STRING(""),
+                           element->OwnerDoc(),
+                           nsComputedDOMStyle::eAll);
+  aRv = cs->GetPropertyValue(aPropertyName, aResult);
+}
+
 } // namespace dom
 } // namespace mozilla
diff --git a/dom/base/AnonymousContent.h b/dom/base/AnonymousContent.h
index fd3b59c440..b56c145956 100644
--- a/dom/base/AnonymousContent.h
+++ b/dom/base/AnonymousContent.h
@@ -68,6 +68,11 @@ public:
                                 const Sequence<OwningNonNull<DOMRect>>& aRects,
                                 ErrorResult& aError);
 
+  void GetComputedStylePropertyValue(const nsAString& aElementId,
+                                     const nsAString& aPropertyName,
+                                     DOMString& aResult,
+                                     ErrorResult& aRv);
+
 private:
   ~AnonymousContent();
   nsCOMPtr<Element> mContentNode;
diff --git a/dom/webidl/AnonymousContent.webidl b/dom/webidl/AnonymousContent.webidl
index 6755fe598a..8be69cd26f 100644
--- a/dom/webidl/AnonymousContent.webidl
+++ b/dom/webidl/AnonymousContent.webidl
@@ -77,4 +77,12 @@ interface AnonymousContent {
   [Throws]
   void setCutoutRectsForElement(DOMString elementId,
                                 sequence<DOMRect> rects);
+
+  /**
+   * Get the computed value of a property on an element inside this custom
+   * anonymous content.
+   */
+  [Throws]
+  DOMString? getComputedStylePropertyValue(DOMString elementId,
+                                           DOMString propertyName);
 };
diff --git a/layout/base/nsCSSFrameConstructor.cpp b/layout/base/nsCSSFrameConstructor.cpp
index 3747642034..3be7c2a0bb 100644
--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -4192,13 +4192,28 @@ nsCSSFrameConstructor::GetAnonymousContent(nsIContent* aParent,
 
     ConnectAnonymousTreeDescendants(content, aContent[i].mChildren);
 
-    // least-surprise CSS binding until we do the SVG specified
-    // cascading rules for <svg:use> - bug 265894
-    if (aParentFrame->GetType() == nsGkAtoms::svgUseFrame) {
+    nsIAtom* parentFrameType = aParentFrame->GetType();
+    if (parentFrameType == nsGkAtoms::svgUseFrame) {
+      // least-surprise CSS binding until we do the SVG specified
+      // cascading rules for <svg:use> - bug 265894
       content->SetFlags(NODE_IS_ANONYMOUS_ROOT);
     } else {
       content->SetIsNativeAnonymousRoot();
-      SetNativeAnonymousBitOnDescendants(content);
+      // Don't mark descendants of the custom content container
+      // as native anonymous.  When canvas custom content is initially
+      // created and appended to the custom content container, in
+      // nsIDocument::InsertAnonymousContent, it is not considered native
+      // anonymous content.  But if we end up reframing the root element,
+      // we will re-create the nsCanvasFrame, and we would end up in here,
+      // marking it as NAC.  Existing uses of canvas custom content would
+      // break if it becomes NAC (since each element starts inheriting
+      // styles from its closest non-NAC ancestor, rather than from its
+      // parent).
+      if (!(parentFrameType == nsGkAtoms::canvasFrame &&
+              content == static_cast<nsCanvasFrame*>(aParentFrame)
+                           ->GetCustomContentContainer())) {
+        SetNativeAnonymousBitOnDescendants(content);
+      }
     }
 
     bool anonContentIsEditable = content->HasFlag(NODE_IS_EDITABLE);
diff --git a/layout/style/test/chrome/chrome.ini b/layout/style/test/chrome/chrome.ini
index e34fce671a..dd3bdf8f56 100644
--- a/layout/style/test/chrome/chrome.ini
+++ b/layout/style/test/chrome/chrome.ini
@@ -10,6 +10,7 @@ support-files =
   mismatch.png
 
 [test_author_specified_style.html]
+[test_bug1346623.html]
 [test_bug418986-2.xul]
 [test_bug1157097.html]
 [test_bug1160724.xul]
diff --git a/layout/style/test/chrome/test_bug1346623.html b/layout/style/test/chrome/test_bug1346623.html
new file mode 100644
index 0000000000..d24d666469
--- /dev/null
+++ b/layout/style/test/chrome/test_bug1346623.html
@@ -0,0 +1,60 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <title>Test for bug 1346623</title>
+  <script type="application/javascript" src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <script type="application/javascript" src="chrome://mochikit/content/tests/SimpleTest/EventUtils.js"></script>
+  <link rel="stylesheet" type="text/css" href="chrome://mochikit/content/tests/SimpleTest/test.css"/>
+</head>
+<body onload="startTest();">
+<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1346623">Mozilla Bug 1346623</a>
+<div id="display">
+
+</div>
+<pre id="test">
+<script type="application/javascript">
+
+var Ci = Components.interfaces;
+var winUtils = window.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowUtils);
+
+function startTest() {
+  // load some styles at the agent level
+  var css = `
+    #ac-parent { color: green; }
+    #ac-child.abc { }
+  `;
+  var sheetURL = "data:text/css," + encodeURIComponent(css);
+  winUtils.loadSheetUsingURIString(sheetURL, winUtils.AGENT_SHEET);
+
+  // add canvas anonymous content
+  var bq = document.createElement("blockquote");
+  bq.id = "ac-parent";
+  bq.textContent = "This blockquote text should be green.";
+  var div = document.createElement("div");
+  div.id = "ac-child";
+  div.textContent = " This div text should be green.";
+  bq.appendChild(div);
+  var ac = document.insertAnonymousContent(bq);
+  document.body.offsetWidth;
+
+  is(ac.getComputedStylePropertyValue("ac-child", "color"), "rgb(0, 128, 0)",
+     "color before reframing");
+
+  // reframe the root
+  document.documentElement.style.display = "flex";
+  document.body.offsetWidth;
+
+  // restyle the div
+  ac.setAttributeForElement("ac-child", "class", "abc");
+  document.body.offsetWidth;
+
+  is(ac.getComputedStylePropertyValue("ac-child", "color"), "rgb(0, 128, 0)",
+     "color after reframing");
+  SimpleTest.finish();
+}
+
+SimpleTest.waitForExplicitFinish();
+</script>
+</pre>
+</body>
+</html>