CSS :has for Accessible Form States

/* Markup contract: each .form__field contains a label, an input,
   an optional .form__hint, and a .form__error paragraph. */

.form {
  display: grid;
  gap: var(--space-4, 1.5rem);
  max-inline-size: 32rem;
}

.form__field {
  display: grid;
  gap: var(--space-2, 0.5rem);
  border-radius: var(--radius-sm, 0.375rem);
}

.form__field > label {
  font-weight: 600;
}

.form__field input {
  font: inherit;
  padding-block: 0.5rem;
  padding-inline: 0.65rem;
  border: 1px solid var(--color-border, #a1a1aa);
  border-radius: var(--radius-sm, 0.375rem);
  background: var(--color-surface, #ffffff);
  color: var(--color-text, #18181b);
}

.form__error {
  display: none;
  margin: 0;
  font-size: 0.875rem;
  color: var(--color-danger, #b91c1c);
}

.form__field:has(input:required) > label::after {
  content: " *";
  color: var(--color-danger, #b91c1c);
}

.form__field:has(input:focus-visible) {
  outline: 2px solid var(--color-focus, #4f46e5);
  outline-offset: 4px;
}

/* :user-invalid appears only after the user has interacted or tried to submit. */
.form__field:has(input:user-invalid) input,
.form__field:has(input[aria-invalid="true"]) input {
  border-color: var(--color-danger, #b91c1c);
  background: var(--color-danger-surface, #fef2f2);
}

.form__field:has(input:user-invalid) .form__error,
.form__field:has(input[aria-invalid="true"]) .form__error {
  display: block;
}

@media (forced-colors: active) {
  .form__field:has(input:user-invalid) input,
  .form__field:has(input[aria-invalid="true"]) input {
    border: 2px solid Mark;
  }
}

/* Fallback without :has: use the general sibling combinator and show the
   message only for inputs explicitly marked invalid by script or server. */
@supports not selector(:has(*)) {
  .form__field input[aria-invalid="true"] {
    border-color: var(--color-danger, #b91c1c);
  }

  .form__field input[aria-invalid="true"] ~ .form__error {
    display: block;
  }
}