:focus-within伪类选择器与表单输入勿扰模式实例页面

代码

CSS代码：

form {
    transition: outline .2s;
    outline: 2000px solid hsla(0,0%,100%,0);
    position: relative;
    z-index: 1;
}
form:focus-within {
    outline: 2000px solid hsla(0,0%,100%,1);
}