純js簡單的表單驗證

<header>
        <div class="left"><img src="./image/baidu.jpg"><i>|</i>
            <em>注冊百度賬号</em>
        </div>
        <div class="right">我已注冊、現在就 <a href="#" target="_blank" rel="external nofollow"  target="_blank" rel="external nofollow"  target="_blank" rel="external nofollow"  target="_blank" rel="external nofollow" >登入</a></div>
    </header>
    <nav></nav>
    <section>
        <form>
            <p>
                <label>使用者名</label>
                <input type="text" id="userName" class="txt1" placeholder="請輸入使用者名">
                <i>設定後可以更改</br>中英文均可最長14個英文或7個漢字</i>
                <span>此使用者名不太受歡迎、請換一個</span>
            </p>
            <p>
                <label>手機号</label>
                <input type="text" id="phone" class="txt1" placeholder="請輸入手機号">
                <i>請輸入手機号、其他使用者不可見</i>
                <span>手機格式不正确、請重新輸入</span>
            </p>
            <p>
                <label>密碼</label>
                <input type="password" id="parsd" class="txt1" placeholder="請輸入密碼">
                <i>長度 為8~4個字元</br>支援數字、大小寫字母标點字元</br>不允許有空格</i>
                <span>格式不正确 你怕是要涼哦！</span>
            </p>
            <p>
                <label>驗證碼</label>
                <input type="text" class="yzm" name="" id="mask" placeholder="請輸入驗證碼" /></label>
                <input type="button" value="擷取短信驗證碼" />
                <!-- <span>驗證碼不正确</span> -->
            </p>
            <p>
                <input type="checkbox" name="" id="" value="" /> 閱讀并接受
                <a href="#" target="_blank" rel="external nofollow"  target="_blank" rel="external nofollow"  target="_blank" rel="external nofollow"  target="_blank" rel="external nofollow" >《1911班協定》</a>及<a href="#" target="_blank" rel="external nofollow"  target="_blank" rel="external nofollow"  target="_blank" rel="external nofollow"  target="_blank" rel="external nofollow" >《1911隐私權保護聲明》</a>
            </p>
            <input type="submit" class="sub" name="" id="" value="注冊" />
        </form>
        <p><a href="#" target="_blank" rel="external nofollow"  target="_blank" rel="external nofollow"  target="_blank" rel="external nofollow"  target="_blank" rel="external nofollow" >如需幫助請點選、人工服務</a></p>
    </section>

</div>