--- a/dom/base/nsStyleLinkElement.cpp
+++ b/dom/base/nsStyleLinkElement.cpp
@@ -85,18 +85,26 @@ nsStyleLinkElement::~nsStyleLinkElement(
   nsStyleLinkElement::SetStyleSheet(nullptr);
 }
 
 void
 nsStyleLinkElement::GetTitleAndMediaForElement(const Element& aSelf,
                                                nsString& aTitle,
                                                nsString& aMedia) const
 {
-  aSelf.GetAttr(kNameSpaceID_None, nsGkAtoms::title, aTitle);
-  aTitle.CompressWhitespace();
+  // Only honor title as stylesheet name for elements in the document (that is,
+  // ignore for Shadow DOM), per [1] and [2]. See [3].
+  //
+  // [1]: https://html.spec.whatwg.org/#attr-link-title
+  // [2]: https://html.spec.whatwg.org/#attr-style-title
+  // [3]: https://github.com/w3c/webcomponents/issues/535
+  if (aSelf.IsInUncomposedDoc()) {
+    aSelf.GetAttr(kNameSpaceID_None, nsGkAtoms::title, aTitle);
+    aTitle.CompressWhitespace();
+  }
 
   aSelf.GetAttr(kNameSpaceID_None, nsGkAtoms::media, aMedia);
   // The HTML5 spec is formulated in terms of the CSSOM spec, which specifies
   // that media queries should be ASCII lowercased during serialization.
   //
   // FIXME(emilio): How does it matter? This is going to be parsed anyway, CSS
   // should take care of serializing it properly.
   nsContentUtils::ASCIIToLower(aMedia);

--- a/testing/web-platform/meta/MANIFEST.json
+++ b/testing/web-platform/meta/MANIFEST.json
@@ -126650,16 +126650,28 @@
       [
        "/css/css-scoping/slotted-with-pseudo-element-ref.html",
        "=="
       ]
      ],
      {}
     ]
    ],
+   "css/css-scoping/stylesheet-title-001.html": [
+    [
+     "/css/css-scoping/stylesheet-title-001.html",
+     [
+      [
+       "/css/css-scoping/reference/green-box.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
    "css/css-shapes/shape-outside/formatting-context/shape-outside-formatting-context.tentative.html": [
     [
      "/css/css-shapes/shape-outside/formatting-context/shape-outside-formatting-context.tentative.html",
      [
       [
        "/css/css-shapes/shape-outside/formatting-context/reference/shape-outside-formatting-context-ref.html",
        "=="
       ]
@@ -317675,16 +317687,22 @@
     ]
    ],
    "css/css-scoping/slotted-slot.html": [
     [
      "/css/css-scoping/slotted-slot.html",
      {}
     ]
    ],
+   "css/css-scoping/stylesheet-title-002.html": [
+    [
+     "/css/css-scoping/stylesheet-title-002.html",
+     {}
+    ]
+   ],
    "css/css-scroll-anchoring/abspos-containing-block-outside-scroller.html": [
     [
      "/css/css-scroll-anchoring/abspos-containing-block-outside-scroller.html",
      {}
     ]
    ],
    "css/css-scroll-anchoring/abspos-contributes-to-static-parent-bounds.html": [
     [
@@ -514754,16 +514772,24 @@
   "css/css-scoping/slotted-with-pseudo-element-ref.html": [
    "48561a3dff973b7ad1bfa9702461e50fd4a67c2d",
    "support"
   ],
   "css/css-scoping/slotted-with-pseudo-element.html": [
    "27d36ba54623bbee2cdd09b7a9322873d5ab0011",
    "reftest"
   ],
+  "css/css-scoping/stylesheet-title-001.html": [
+   "2e49b6d74d2021144444ca77e62acbc6aeffac2a",
+   "reftest"
+  ],
+  "css/css-scoping/stylesheet-title-002.html": [
+   "9e228381a0aa11c76568b8ec893ed6745581bbf9",
+   "testharness"
+  ],
   "css/css-scroll-anchoring/README.md": [
    "31205944cbcf321f7aa77e3bef0f8835cc7b6d13",
    "support"
   ],
   "css/css-scroll-anchoring/abspos-containing-block-outside-scroller.html": [
    "d7a8e9904637c833d897b2e9c0da0a1628455670",
    "testharness"
   ],
@@ -553639,17 +553665,17 @@
    "40ab5636653dfd105738ab38e7d22316132eb630",
    "testharness"
   ],
   "dom/events/event-global.worker.js": [
    "084a6f752edee6578113035fece6d0eb85a2fdf7",
    "testharness"
   ],
   "dom/events/relatedTarget.window.js": [
-   "0426d2ecae3f3562be175e4364353d979365ed1c",
+   "3dd9ddfa4d8d584f6ddf6db52e4e9020491088d8",
    "testharness"
   ],
   "dom/historical.html": [
    "291d078c4424d0491aeab7524d350eab7304f989",
    "testharness"
   ],
   "dom/interface-objects.html": [
    "05963c8b0a839f3e07b5478d99fc034e4654b515",
@@ -579759,17 +579785,17 @@
    "da39a3ee5e6b4b0d3255bfef95601890afd80709",
    "support"
   ],
   "html/webappapis/timers/evil-spec-example.html": [
    "49fd55dbbf64c6973a0e76284c0e3d8b7bf0ef3c",
    "testharness"
   ],
   "html/webappapis/timers/missing-timeout-setinterval.any.js": [
-   "79b4a278f0e35646cfdffeebf8f0523e2772bc9b",
+   "40ad74d6ed3719dcad2097246d74d49c87b989aa",
    "testharness"
   ],
   "html/webappapis/timers/negative-setinterval.html": [
    "405046cab9cd15a88d57eace1f293ebdd7b1b3e2",
    "testharness"
   ],
   "html/webappapis/timers/negative-settimeout.html": [
    "e5673e7cca2b006afd3e2e4e5dd3e56fb10efa4e",

new file mode 100644
--- /dev/null
+++ b/testing/web-platform/tests/css/css-scoping/stylesheet-title-001.html
@@ -0,0 +1,24 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: title attribute in stylesheets not in the document tree is ignored</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="help" href="https://drafts.csswg.org/cssom/#preferred-css-style-sheet-set-name">
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#attr-style-title">
+<link rel="help" href="https://github.com/w3c/webcomponents/issues/535">
+<link rel="match" href="reference/green-box.html">
+<p>Test passes if you see a single 100px by 100px green box below.</p>
+<div id="host"></div>
+<script>
+  host.attachShadow({ mode: "open" }).innerHTML = `
+    <style>
+      div { width: 100px; height: 100px; }
+    </style>
+    <style title="Foo">
+      div { background: purple }
+    </style>
+    <style title="Bar">
+      div { background: green }
+    </style>
+    <div></div>
+  `;
+</script>

new file mode 100644
--- /dev/null
+++ b/testing/web-platform/tests/css/css-scoping/stylesheet-title-002.html
@@ -0,0 +1,29 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>CSS Test: title attribute in stylesheets not in the document tree is ignored</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="help" href="https://drafts.csswg.org/cssom/#preferred-css-style-sheet-set-name">
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#attr-style-title">
+<link rel="help" href="https://github.com/w3c/webcomponents/issues/535">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="host"></div>
+<script>
+test(function() {
+  host.attachShadow({ mode: "open" }).innerHTML = `
+    <style>
+      div { width: 100px; height: 100px; }
+    </style>
+    <style title="Foo">
+      div { background: purple }
+    </style>
+    <style title="Bar">
+      div { background: green }
+    </style>
+    <div></div>
+  `;
+  assert_equals(host.shadowRoot.styleSheets.length, 3);
+  for (let sheet of host.shadowRoot.styleSheets)
+    assert_equals(sheet.title, null);
+}, "Title attribute in stylesheets not in the document tree is ignored");
+</script>