diff --git a/src/browser/tests/document/query_selector_all.html b/src/browser/tests/document/query_selector_all.html
index aa56b285..c98de73c 100644
--- a/src/browser/tests/document/query_selector_all.html
+++ b/src/browser/tests/document/query_selector_all.html
@@ -376,3 +376,67 @@
 }
 </script>
 
+<form id="form-validity-test">
+  <input id="vi-required-empty" type="text" required>
+  <input id="vi-optional" type="text">
+  <input id="vi-hidden-required" type="hidden" required>
+  <fieldset id="vi-fieldset">
+    <input id="vi-nested-required" type="text" required>
+    <select id="vi-select-required" required>
+      <option value="">Pick one</option>
+      <option value="a">A</option>
+    </select>
+  </fieldset>
+</form>
+<input id="vi-checkbox" type="checkbox">
+
+<script id=invalidPseudo>
+{
+  // Inputs with required + empty value are :invalid
+  testing.expectEqual(true, document.getElementById('vi-required-empty').matches(':invalid'));
+  testing.expectEqual(false, document.getElementById('vi-required-empty').matches(':valid'));
+
+  // Inputs without required are :valid
+  testing.expectEqual(false, document.getElementById('vi-optional').matches(':invalid'));
+  testing.expectEqual(true, document.getElementById('vi-optional').matches(':valid'));
+
+  // hidden inputs are not candidates for constraint validation
+  testing.expectEqual(false, document.getElementById('vi-hidden-required').matches(':invalid'));
+  testing.expectEqual(false, document.getElementById('vi-hidden-required').matches(':valid'));
+
+  // select with required and empty selected value is :invalid
+  testing.expectEqual(true, document.getElementById('vi-select-required').matches(':invalid'));
+  testing.expectEqual(false, document.getElementById('vi-select-required').matches(':valid'));
+
+  // fieldset containing invalid controls is :invalid
+  testing.expectEqual(true, document.getElementById('vi-fieldset').matches(':invalid'));
+  testing.expectEqual(false, document.getElementById('vi-fieldset').matches(':valid'));
+
+  // form containing invalid controls is :invalid
+  testing.expectEqual(true, document.getElementById('form-validity-test').matches(':invalid'));
+  testing.expectEqual(false, document.getElementById('form-validity-test').matches(':valid'));
+}
+</script>
+
+<script id=validAfterValueSet>
+{
+  // After setting a value, a required input becomes :valid
+  const input = document.getElementById('vi-required-empty');
+  input.value = 'hello';
+  testing.expectEqual(false, input.matches(':invalid'));
+  testing.expectEqual(true, input.matches(':valid'));
+  input.value = '';
+}
+</script>
+
+<script id=indeterminatePseudo>
+{
+  const cb = document.getElementById('vi-checkbox');
+  testing.expectEqual(false, cb.matches(':indeterminate'));
+  cb.indeterminate = true;
+  testing.expectEqual(true, cb.matches(':indeterminate'));
+  cb.indeterminate = false;
+  testing.expectEqual(false, cb.matches(':indeterminate'));
+}
+</script>
+
diff --git a/src/browser/webapi/Element.zig b/src/browser/webapi/Element.zig
index 5682b01d..ec533d9c 100644
--- a/src/browser/webapi/Element.zig
+++ b/src/browser/webapi/Element.zig
@@ -958,7 +958,7 @@ pub fn closest(self: *Element, selector: []const u8, page: *Page) !?*Element {
 
     var current: ?*Element = self;
     while (current) |el| {
-        if (try el.matches(selector, page)) {
+        if (try Selector.matchesWithScope(el, selector, self, page)) {
             return el;
         }
 
diff --git a/src/browser/webapi/selector/List.zig b/src/browser/webapi/selector/List.zig
index ec2d90d3..3ed295c4 100644
--- a/src/browser/webapi/selector/List.zig
+++ b/src/browser/webapi/selector/List.zig
@@ -531,11 +531,45 @@ fn matchesPseudoClass(el: *Node.Element, pseudo: Selector.PseudoClass, scope: *N
         .enabled => {
             return el.getAttributeSafe(comptime .wrap("disabled")) == null;
         },
-        .indeterminate => return false,
+        .indeterminate => {
+            const input = el.is(Node.Element.Html.Input) orelse return false;
+            return switch (input._input_type) {
+                .checkbox => input.getIndeterminate(),
+                else => false,
+            };
+        },
 
         // Form validation
-        .valid => return false,
-        .invalid => return false,
+        .valid => {
+            if (el.is(Node.Element.Html.Input)) |input| {
+                return switch (input._input_type) {
+                    .hidden, .submit, .reset, .button => false,
+                    else => !input.getRequired() or input.getValue().len > 0,
+                };
+            }
+            if (el.is(Node.Element.Html.Select)) |select| {
+                return !select.getRequired() or select.getValue(page).len > 0;
+            }
+            if (el.is(Node.Element.Html.Form) != null or el.is(Node.Element.Html.FieldSet) != null) {
+                return !hasInvalidDescendant(node, page);
+            }
+            return false;
+        },
+        .invalid => {
+            if (el.is(Node.Element.Html.Input)) |input| {
+                return switch (input._input_type) {
+                    .hidden, .submit, .reset, .button => false,
+                    else => input.getRequired() and input.getValue().len == 0,
+                };
+            }
+            if (el.is(Node.Element.Html.Select)) |select| {
+                return select.getRequired() and select.getValue(page).len == 0;
+            }
+            if (el.is(Node.Element.Html.Form) != null or el.is(Node.Element.Html.FieldSet) != null) {
+                return hasInvalidDescendant(node, page);
+            }
+            return false;
+        },
         .required => {
             return el.getAttributeSafe(comptime .wrap("required")) != null;
         },
@@ -684,6 +718,26 @@ fn matchesHasDescendant(el: *Node.Element, selector: Selector.Selector, scope: *
     return false;
 }
 
+fn hasInvalidDescendant(parent: *Node, page: *Page) bool {
+    var child = parent.firstChild();
+    while (child) |c| {
+        if (c.is(Node.Element)) |child_el| {
+            if (child_el.is(Node.Element.Html.Input)) |input| {
+                const invalid = switch (input._input_type) {
+                    .hidden, .submit, .reset, .button => false,
+                    else => input.getRequired() and input.getValue().len == 0,
+                };
+                if (invalid) return true;
+            } else if (child_el.is(Node.Element.Html.Select)) |select| {
+                if (select.getRequired() and select.getValue(page).len == 0) return true;
+            }
+        }
+        if (hasInvalidDescendant(c, page)) return true;
+        child = c.nextSibling();
+    }
+    return false;
+}
+
 fn isFirstChild(el: *Node.Element) bool {
     const node = el.asNode();
     var sibling = node.previousSibling();
diff --git a/src/browser/webapi/selector/Selector.zig b/src/browser/webapi/selector/Selector.zig
index 0ac9793a..4a236fc3 100644
--- a/src/browser/webapi/selector/Selector.zig
+++ b/src/browser/webapi/selector/Selector.zig
@@ -92,6 +92,24 @@ pub fn matches(el: *Node.Element, input: []const u8, page: *Page) !bool {
     return false;
 }
 
+// Like matches, but allows the caller to specify a scope node distinct from el.
+// Used by closest() so that :scope always refers to the original context element.
+pub fn matchesWithScope(el: *Node.Element, input: []const u8, scope: *Node.Element, page: *Page) !bool {
+    if (input.len == 0) {
+        return error.SyntaxError;
+    }
+
+    const arena = page.call_arena;
+    const selectors = try Parser.parseList(arena, input, page);
+
+    for (selectors) |selector| {
+        if (List.matches(el.asNode(), selector, scope.asNode(), page)) {
+            return true;
+        }
+    }
+    return false;
+}
+
 pub fn classAttributeContains(class_attr: []const u8, class_name: []const u8) bool {
     if (class_name.len == 0 or class_name.len > class_attr.len) return false;