由于过于生疏还不太理解，写的不好，请老师谅解！
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>表单css</title><style>/* 样式初始化 reset.css */* {margin: 0;padding: 0;box-sizing: border-box;}a {color: #555;text-decoration: none;}/* 基本选择器 */body{background-color: rgb(0, 162, 255);}/* 上下文选择器 */em + li {background-color: blueviolet;}</style></head><body><!-- 标题 --><h3 align="center">用户登陆</h3><form action=""><!-- 表单分组 --><fieldset align="center" ><ul><li><div><div><label for="username">帐号:</label><inputtype="text"id="username"name="username"autofocusrequiredplaceholder="登陆账户"value="username"/></div></div></li><li><div><label for="psw">密码:</label><input type="password" id="psw" name="psw" required placeholder="不少于6位" /><!-- <button type="button" onclick="document.querySelector('#psw').type='text'">显示密码</button> --></div></li><li class="em"><div><div><label for="email">邮箱:</label><input type="email" id="email" name="email" required placeholder="deml@email.com" class="em"/></div></div></li><li ><button>登陆</button></li></ul></fieldset></form></body></html>
显示效果
