A fieldset is :valid when none of its controls is invalid

src/nwsapi.mts

@@ -2213,7 +2213,7 @@
                     'if(((' +
                     '(/^form$/i.test(e.localName)&&!e.noValidate)||' +
                     '(e.willValidate&&!e.formNoValidate))&&e.checkValidity())||' +
-                    '(/^fieldset$/i.test(e.localName)&&s.first(":valid",e))' +
+                    '(/^fieldset$/i.test(e.localName)&&!s.first(":invalid",e))' +
                     '){' +
                     source +
                     '}'

test/repo/unit/fieldset-valid.test.mts

@@ -0,0 +1,74 @@
+import { expect, test } from 'vitest'
+import { JSDOM } from 'jsdom'
+import factory from '../../../src/nwsapi.js'
+
+test.each([
+  ['empty', '<fieldset id="subject"></fieldset>', true],
+  ['valid control', '<fieldset id="subject"><input></fieldset>', true],
+  [
+    'invalid control',
+    '<fieldset id="subject"><input required></fieldset>',
+    false,
+  ],
+  [
+    'disabled control',
+    '<fieldset id="subject"><input required disabled></fieldset>',
+    true,
+  ],
+  [
+    'disabled ancestor',
+    '<fieldset disabled><fieldset id="subject"><input required></fieldset></fieldset>',
+    true,
+  ],
+  [
+    'nested invalid control',
+    '<fieldset id="subject"><fieldset><input required></fieldset></fieldset>',
+    false,
+  ],
+])('fieldset validity: %s', (_name, html, valid) => {
+  const { window } = new JSDOM(html)
+  try {
+    const engine = factory(window)
+    const subject = window.document.getElementById('subject')!
+    for (let repeat = 0; repeat < 2; repeat += 1) {
+      expect(engine.match(':valid', subject)).toBe(valid)
+      expect(engine.match(':invalid', subject)).toBe(!valid)
+      expect(engine.select('#subject:valid', window.document)).toEqual(
+        valid ? [subject] : [],
+      )
+      expect(engine.select('#subject:invalid', window.document)).toEqual(
+        valid ? [] : [subject],
+      )
+    }
+  } finally {
+    window.close()
+  }
+})
+
+test('cached fieldset queries observe validation and tree changes', () => {
+  const { window } = new JSDOM('<fieldset><input required></fieldset>')
+  try {
+    const engine = factory(window)
+    const subject = window.document.querySelector('fieldset')!
+    const control = window.document.querySelector('input')!
+    const assertValid = (valid: boolean) => {
+      expect(engine.match(':valid', subject)).toBe(valid)
+      expect(engine.select('fieldset:valid', window.document)).toEqual(
+        valid ? [subject] : [],
+      )
+    }
+    assertValid(false)
+    control.value = 'complete'
+    assertValid(true)
+    control.setCustomValidity('invalid')
+    assertValid(false)
+    control.disabled = true
+    assertValid(true)
+    control.disabled = false
+    assertValid(false)
+    control.remove()
+    assertValid(true)
+  } finally {
+    window.close()
+  }
+})