气泡 无边框

<span class="form_hint">正确格式为：6~18个字符，可使用字母、数字、下划线，需以字母开头</span>

.form_hint {
    background: #fff;
    /*border: 1px solid #eee;*/
    border-radius: 3px 3px 3px 3px;
    color: #333;
    margin-left:8px;
    padding: 1px 6px;
    z-index: 999;
    position: absolute;
}
.form_hint::before {
    content: "\25C0";
    background: #fff;
    color:#fff;
    position: absolute;
    top:1px;
    left:-6px;
}