diff --git a/Libraries/LibWeb/HTML/CustomElements/CustomElementName.cpp b/Libraries/LibWeb/HTML/CustomElements/CustomElementName.cpp
index 340fc51f3f1..ad6464839ae 100644
--- a/Libraries/LibWeb/HTML/CustomElements/CustomElementName.cpp
+++ b/Libraries/LibWeb/HTML/CustomElements/CustomElementName.cpp
@@ -1,42 +1,53 @@
 /*
  * Copyright (c) 2023, Srikavin Ramkumar <me@srikavin.me>
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#include <AK/StringView.h>
 #include <AK/Utf8View.h>
+#include <LibWeb/DOM/Element.h>
 #include <LibWeb/HTML/CustomElements/CustomElementName.h>
 
 namespace Web::HTML {
 
-// https://html.spec.whatwg.org/multipage/custom-elements.html#custom-elements-core-concepts:prod-pcenchar
-static bool is_pcen_char(u32 code_point)
-{
-    return code_point == '-'
-        || code_point == '.'
-        || (code_point >= '0' && code_point <= '9')
-        || code_point == '_'
-        || (code_point >= 'a' && code_point <= 'z')
-        || code_point == 0xb7
-        || (code_point >= 0xc0 && code_point <= 0xd6)
-        || (code_point >= 0xd8 && code_point <= 0xf6)
-        || (code_point >= 0xf8 && code_point <= 0x37d)
-        || (code_point >= 0x37f && code_point <= 0x1fff)
-        || (code_point >= 0x200c && code_point <= 0x200d)
-        || (code_point >= 0x203f && code_point <= 0x2040)
-        || (code_point >= 0x2070 && code_point <= 0x218f)
-        || (code_point >= 0x2c00 && code_point <= 0x2fef)
-        || (code_point >= 0x3001 && code_point <= 0xD7ff)
-        || (code_point >= 0xf900 && code_point <= 0xfdcf)
-        || (code_point >= 0xfdf0 && code_point <= 0xfffd)
-        || (code_point >= 0x10000 && code_point <= 0xeffff);
-}
-
 // https://html.spec.whatwg.org/multipage/custom-elements.html#valid-custom-element-name
-bool is_valid_custom_element_name(StringView name)
+bool is_valid_custom_element_name(String const& name)
 {
-    // name must not be any of the following:
+    // A string name is a valid custom element name if all of the following are true:
+    // - name is a valid element local name;
+    if (!DOM::is_valid_element_local_name(name))
+        return false;
+
+    // - name's 0th code point is an ASCII lower alpha;
+    auto code_points = Utf8View { name };
+    if (auto first = code_points.begin(); first.done() || !is_ascii_lower_alpha(*first))
+        return false;
+
+    // - name does not contain any ASCII upper alphas;
+    // - name contains a U+002D (-); and
+    bool contains_ascii_upper_alpha = false;
+    bool contains_hyphen = false;
+    for (auto code_point : code_points) {
+        if (is_ascii_upper_alpha(code_point)) {
+            contains_ascii_upper_alpha = true;
+            break;
+        }
+        if (code_point == '-')
+            contains_hyphen = true;
+    }
+    if (contains_ascii_upper_alpha || !contains_hyphen)
+        return false;
+
+    // - name is not one of the following:
+    //   - "annotation-xml"
+    //   - "color-profile"
+    //   - "font-face"
+    //   - "font-face-src"
+    //   - "font-face-uri"
+    //   - "font-face-format"
+    //   - "font-face-name"
+    //   - "missing-glyph"
     if (name.is_one_of(
             "annotation-xml"sv,
             "color-profile"sv,
@@ -49,28 +60,7 @@ bool is_valid_custom_element_name(StringView name)
         return false;
     }
 
-    // name must match the PotentialCustomElementName production:
-    // PotentialCustomElementName ::=
-    //      [a-z] (PCENChar)* '-' (PCENChar)*
-
-    auto code_points = Utf8View { name };
-    auto it = code_points.begin();
-
-    if (code_points.is_empty() || *it < 'a' || *it > 'z')
-        return false;
-
-    ++it;
-
-    bool found_hyphen = false;
-
-    for (; it != code_points.end(); ++it) {
-        if (!is_pcen_char(*it))
-            return false;
-        if (*it == '-')
-            found_hyphen = true;
-    }
-
-    return found_hyphen;
+    return true;
 }
 
 }
diff --git a/Libraries/LibWeb/HTML/CustomElements/CustomElementName.h b/Libraries/LibWeb/HTML/CustomElements/CustomElementName.h
index b16ea325cdd..a8090565452 100644
--- a/Libraries/LibWeb/HTML/CustomElements/CustomElementName.h
+++ b/Libraries/LibWeb/HTML/CustomElements/CustomElementName.h
@@ -1,15 +1,21 @@
 /*
  * Copyright (c) 2023, Srikavin Ramkumar <me@srikavin.me>
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #pragma once
 
-#include <AK/StringView.h>
+#include <AK/String.h>
 
 namespace Web::HTML {
 
-bool is_valid_custom_element_name(StringView name);
+bool is_valid_custom_element_name(String const& name);
+
+inline bool is_valid_custom_element_name(FlyString const& name)
+{
+    return is_valid_custom_element_name(name.to_string());
+}
 
 }
diff --git a/Tests/LibWeb/Text/expected/wpt-import/custom-elements/registries/valid-custom-element-names.txt b/Tests/LibWeb/Text/expected/wpt-import/custom-elements/registries/valid-custom-element-names.txt
new file mode 100644
index 00000000000..4dc28b04921
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/custom-elements/registries/valid-custom-element-names.txt
@@ -0,0 +1,6 @@
+Harness status: OK
+
+Found 1 tests
+
+1 Pass
+Pass	valid-custom-element-names
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/input/wpt-import/custom-elements/registries/valid-custom-element-names.html b/Tests/LibWeb/Text/input/wpt-import/custom-elements/registries/valid-custom-element-names.html
new file mode 100644
index 00000000000..4bd33c79099
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/custom-elements/registries/valid-custom-element-names.html
@@ -0,0 +1,138 @@
+<!DOCTYPE html>
+<link rel=author href="mailto:jarhar@chromium.org">
+<link rel=help href="https://github.com/whatwg/html/pull/7991">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+
+<script>
+function isAsciiLowerAlpha(codePoint) {
+  return codePoint >= 0x61 && codePoint <= 0x7A;
+}
+function isAsciiUpperAlpha(codePoint) {
+  return codePoint >= 0x41 && codePoint <= 0x5A;
+}
+function isAsciiAlpha(codePoint) {
+  return isAsciiLowerAlpha(codePoint) || isAsciiUpperAlpha(codePoint);
+}
+function isAsciiDigit(codePoint) {
+  return codePoint >= 0x30 && codePoint <= 0x39;
+}
+function isAsciiWhitespace(codePoint) {
+  return codePoint == 0x9 || codePoint == 0xA || codePoint == 0xC || codePoint == 0xD || codePoint == 0x20;
+}
+
+function debugString(str) {
+  const codePoints = [];
+  for (const c of str) {
+    codePoints.push(c.codePointAt(0));
+  }
+  return `code points: ${JSON.stringify(codePoints)}, string: "${str}"`;
+}
+
+const validCustomElementNames = [
+  'annotation-xml-custom',
+];
+const invalidCustomElementNames = [
+  '',
+  'annotation-xml',
+  'color-profile',
+  'font-face',
+  'font-face-src',
+  'font-face-uri',
+  'font-face-format',
+  'font-face-name',
+  'missing-glyph',
+];
+
+const testCodePoints = [0x1F171, 0x1F196, 0x10000];
+for (let i = 0; i < 0x80; i++) {
+  testCodePoints.push(i);
+}
+
+const elementLocalNameRegex = /^(?:[A-Za-z][^\0\t\n\f\r\u0020/>]*|[:_\u0080-\u{10FFFF}][A-Za-z0-9-.:_\u0080-\u{10FFFF}]*)$/u;
+
+function isValidCustomElementName(str) {
+  if (!str.length) {
+    return false;
+  }
+
+  if (!str.includes('-')) {
+    return false;
+  }
+
+  let first = true;
+  for (const c of str) {
+    const codePoint = c.codePointAt(0);
+    if (first) {
+      if (!isAsciiLowerAlpha(codePoint)) {
+        return false;
+      }
+      first = false;
+    }
+    if (isAsciiUpperAlpha(codePoint)) {
+      return false;
+    }
+  }
+
+  return elementLocalNameRegex.test(str);
+}
+
+// In order to test the branching logic of valid element local names and the
+// requirement of having a '-' character, this method generates different
+// variations of potential custom element names given two code points.
+function createStringWithSeparatorMode(codePoint, prefix, separatorMode) {
+  const str = String.fromCodePoint(codePoint);
+  if (separatorMode == 0) {
+    return `${prefix}${str}`;
+  } else if (separatorMode == 1) {
+    return `${prefix}-${str}`;
+  } else if (separatorMode == 2) {
+    return `${prefix}${str}-element`;
+  }
+}
+
+for (const prefix of ['', 'a', 'A', ' ', '\0']) {
+  for (const codePoint of testCodePoints) {
+    for (const separatorMode of [0, 1, 2]) {
+      const str = createStringWithSeparatorMode(
+        codePoint, prefix, separatorMode);
+      if (isValidCustomElementName(str)) {
+        validCustomElementNames.push(str);
+      } else {
+        invalidCustomElementNames.push(str);
+      }
+    }
+  }
+}
+
+let nextClassNumber = 1;
+function createElementClass() {
+  const name = `CustomElement${nextClassNumber++}`;
+  const newClass = function() {};
+  newClass.prototype = HTMLElement;
+  return newClass;
+}
+
+promise_test(async t => {
+  for (const validName of validCustomElementNames) {
+    try {
+      const newClass = createElementClass();
+      customElements.define(validName, newClass);
+      await customElements.whenDefined(validName);
+    } catch (error) {
+      assert_unreached(`Custom element name should have been valid but threw error: ${debugString(validName)} ${error.toString()}`);
+    }
+  }
+
+  for (const invalidName of invalidCustomElementNames) {
+    const newClass = createElementClass();
+    assert_throws_dom(
+      'SyntaxError',
+      () => customElements.define(invalidName, newClass),
+      `customElements.define should have thrown for invalid name: ${debugString(invalidName)}`);
+    await promise_rejects_dom(t, 'SyntaxError',
+      customElements.whenDefined(invalidName),
+      `customElements.whenDefined should have thrown for invalid name: ${debugString(invalidName)}`);
+  }
+});
+</script>