diff --git a/Libraries/LibWeb/HTML/HTMLElement.cpp b/Libraries/LibWeb/HTML/HTMLElement.cpp
index 55df08802c2..499b4468a6a 100644
--- a/Libraries/LibWeb/HTML/HTMLElement.cpp
+++ b/Libraries/LibWeb/HTML/HTMLElement.cpp
@@ -852,13 +852,15 @@ GC::Ptr<DOM::NodeList> HTMLElement::labels()
     return m_labels;
 }
 
+// https://html.spec.whatwg.org/multipage/interaction.html#dom-hidden
 Variant<bool, double, String> HTMLElement::hidden() const
 {
     // 1. If the hidden attribute is in the hidden until found state, then return "until-found".
-    if (get_attribute(HTML::AttributeNames::hidden) == "until-found")
+    auto const& hidden = get_attribute(HTML::AttributeNames::hidden);
+    if (hidden.has_value() && hidden->equals_ignoring_ascii_case("until-found"sv))
         return "until-found"_string;
     // 2. If the hidden attribute is set, then return true.
-    if (has_attribute(HTML::AttributeNames::hidden))
+    if (hidden.has_value())
         return true;
     // 3. Return false.
     return false;
diff --git a/Tests/LibWeb/Text/expected/wpt-import/html/editing/the-hidden-attribute/hidden-ua-stylesheet.txt b/Tests/LibWeb/Text/expected/wpt-import/html/editing/the-hidden-attribute/hidden-ua-stylesheet.txt
new file mode 100644
index 00000000000..dde9413ee0d
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/html/editing/the-hidden-attribute/hidden-ua-stylesheet.txt
@@ -0,0 +1,13 @@
+Harness status: OK
+
+Found 8 tests
+
+8 Pass
+Pass	hidden-ua-stylesheet
+Pass	div.removeAttribute('hidden')
+Pass	div.setAttribute('hidden', '')
+Pass	div.setAttribute('hidden', 'asdf')
+Pass	div.setAttribute('hidden', 'until-found')
+Pass	div.setAttribute('hidden', 'UNTIL-FOUND')
+Pass	div.setAttribute('hidden', 'UnTiL-FoUnD')
+Pass	div.setAttribute('hidden', '0')
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/html/editing/the-hidden-attribute/hidden-until-found-idl.txt b/Tests/LibWeb/Text/expected/wpt-import/html/editing/the-hidden-attribute/hidden-until-found-idl.txt
new file mode 100644
index 00000000000..f0f6a0e9cdc
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/html/editing/the-hidden-attribute/hidden-until-found-idl.txt
@@ -0,0 +1,6 @@
+Harness status: OK
+
+Found 1 tests
+
+1 Pass
+Pass	element.hidden should return "until-found" regardless of uppercase letters.
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/editing/the-hidden-attribute/hidden-ua-stylesheet.html b/Tests/LibWeb/Text/input/wpt-import/html/editing/the-hidden-attribute/hidden-ua-stylesheet.html
new file mode 100644
index 00000000000..a7bddc3f727
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/editing/the-hidden-attribute/hidden-ua-stylesheet.html
@@ -0,0 +1,68 @@
+<!DOCTYPE html>
+<link rel=author href="mailto:jarhar@chromium.org">
+<link rel=help href="https://html.spec.whatwg.org/multipage/rendering.html#hiddenCSS">
+<link rel=help href="https://github.com/whatwg/html/pull/7475#issuecomment-1069313217">
+<script src="../../../resources/testharness.js"></script>
+<script src="../../../resources/testharnessreport.js"></script>
+
+<div id=div>hello world</div>
+<table id=table>
+  <colgroup id=colgroup>
+    <col id=col></col>
+  </colgroup>
+</table>
+
+<script>
+function testDisplayNone(description) {
+  test(() => {
+    assert_equals(getComputedStyle(div).display, 'none',
+        `${description} should make the div display:none.`);
+    assert_equals(getComputedStyle(div).contentVisibility, 'visible',
+        `${description} should not affect the div's content-visibility property.`);
+  }, description);
+}
+
+function testCVHidden(description) {
+  test(() => {
+    assert_equals(getComputedStyle(div).display, 'block',
+        `${description} should not affect the div's display property.`);
+    assert_equals(getComputedStyle(div).contentVisibility, 'hidden',
+        `${description} should make the div content-visibility:hidden.`);
+    assert_equals(div.hidden, "until-found",
+      `${description} should make the div hidden=until-found.`);
+  }, description);
+
+}
+
+function testNormal(description) {
+  test(() => {
+    assert_equals(getComputedStyle(div).display, 'block',
+        `${description} should not affect the div's display property.`);
+    assert_equals(getComputedStyle(div).contentVisibility, 'visible',
+        `${description} should not affect the div's content-visibility property.`);
+  }, description);
+}
+
+test(() => {
+  div.removeAttribute('hidden');
+  testNormal(`div.removeAttribute('hidden')`);
+
+  div.setAttribute('hidden', '');
+  testDisplayNone(`div.setAttribute('hidden', '')`);
+
+  div.setAttribute('hidden', 'asdf');
+  testDisplayNone(`div.setAttribute('hidden', 'asdf')`);
+
+  div.setAttribute('hidden', 'until-found');
+  testCVHidden(`div.setAttribute('hidden', 'until-found')`);
+
+  div.setAttribute('hidden', 'UNTIL-FOUND');
+  testCVHidden(`div.setAttribute('hidden', 'UNTIL-FOUND')`);
+
+  div.setAttribute('hidden', 'UnTiL-FoUnD');
+  testCVHidden(`div.setAttribute('hidden', 'UnTiL-FoUnD')`);
+
+  div.setAttribute('hidden', '0');
+  testDisplayNone(`div.setAttribute('hidden', '0')`);
+});
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/editing/the-hidden-attribute/hidden-until-found-idl.html b/Tests/LibWeb/Text/input/wpt-import/html/editing/the-hidden-attribute/hidden-until-found-idl.html
new file mode 100644
index 00000000000..5df0b767fd6
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/editing/the-hidden-attribute/hidden-until-found-idl.html
@@ -0,0 +1,20 @@
+<!DOCTYPE html>
+<link rel=author href="mailto:jarhar@chromium.org">
+<link rel=help href="https://html.spec.whatwg.org/#the-hidden-attribute">
+<link rel=helph href="https://issues.chromium.org/issues/402108887">
+<script src="../../../resources/testharness.js"></script>
+<script src="../../../resources/testharnessreport.js"></script>
+
+<div id=target>div</div>
+
+<script>
+test(() => {
+  const target = document.getElementById('target');
+  target.setAttribute('hidden', 'UNTIL-FOUND');
+  assert_equals(target.hidden, 'until-found');
+  target.setAttribute('hidden', 'uNtIl-FoUnD');
+  assert_equals(target.hidden, 'until-found');
+  target.setAttribute('hidden', 'until-found');
+  assert_equals(target.hidden, 'until-found');
+}, 'element.hidden should return "until-found" regardless of uppercase letters.');
+</script>
