1 举例说明状态伪类

<style>
        form{
           margin-left: 36%;
        }
        fieldset{
            width: fit-content;
            background:
            radial-gradient(circle at 50% 0,
                    rgba(255,0,0,.5),
                    rgba(255,0,0,0) 70.71%),
                radial-gradient(circle at 6.7% 75%,
                    rgba(0,0,255,.5),
                    rgba(0,0,255,0) 70.71%);          
        }
        div{
            margin: 10px;
        }
        div span{
            color: red;
        }
        .tijiao{
            text-align: center;
        }

必填

        /* 必填 */
        input:required {
        border: 1px solid black;
        }

效果如下：

被选中

        /* 被选中 */
        input:focus {
        border: 1px solid silver;
        }

悬停

        /* 鼠标在按钮时悬停切换成手势图标,字体变粗,颜色变成棕色 */
        div button:hover{
            color: brown;
            font-weight: bolder;
            cursor:pointer;
        }
    </style>

    <form accept="register.php" method="post">
        <fieldset>
          <legend>用户注册</legend>
            <!-- 用户名 -->
          <div>
            <label for="username">用&nbsp;&nbsp;户&nbsp;&nbsp;名: </label>
            <input id="username" name="username" type="text" required />
            <span >*</span>
          </div>
          <!-- 密码 -->
          <div>
            <label for="password">密 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 码: </label>
            <input id="password" name="password" type="text" required /> 
            <span >*</span>
          </div>
          <!-- 确认密码 -->
          <div>
            <label for="repwd">确认密码: </label>
            <input id="repwd" name="repwd" type="text" required />
            <span >*</span>
          </div>
          <div>
            <label for="email">
              用户邮箱:
            </label>
            <input id="email" name="email" type="email" />
            <span >*</span>
          </div>
          <div class="tijiao">
            <button>注册</button>
            <button>取消</button>
        </div>
        </fieldset>
      </form>

盒模型

2.1 盒模型的五个核心属性：

width —- 宽度
height —- 高度
border —- 边框
margin —- 外边距
padding —-内边距

    <style>
        .box{
            width: 200px;
            height: 120px;
            margin: 30px;
            padding: 30px;
            border: 5px solid rgb(152, 46, 46);
        }
    </style>
    <div class="box"></div>