表单：用户行为规范
使服务器成为表单有效性的权威，而客户端则为了速度进行验证，共享错误代码，聚焦第一个无效字段，并且绝不泄露私有记录是否存在的信息。
Defines keyboard, focus, filtering, selection, and announcement behavior for a combobox pattern aligned with WAI-ARIA Authoring Practices without claiming WCAG conformance by default.