css
/*QuickReset*/*{box-sizing:border-box;margin:0;}html,body{height:100%;}
body{
background:url(https://gimg2.baidu.com/image_search/src=http%3A%2F%2Flmg.jj20.com%2Fup%2Fallimg%2F1114%2F033021091503%2F210330091503-6-1200.jpg&refer=http%3A%2F%2Flmg.jj20.com&app=2002&size=f9999,10000&q=a80&n=0&g=0n&fmt=auto?sec=1674264288&t=7b029187a517d437eda7cec95e2cb04a) 50%;
background-size: cover;
}
#aside{ /* your ex-blue */
position:fixed;
overflow:hidden; /* to contain the child's blue box shadow */
width: 100%;
height: 100%;
right: 0;
top: 0;
color:#fff;
border:1px solid red;
}
/* your ex-yellow... hole... something */
#hole{
width: 300px;
height: 300px;
box-shadow: 0 0 0 300vh rgba(0,0,0,0.3); /* huge blue box shadow */
border-radius:50%;
position:absolute;
top: 21%;
left: 54%;
}
.center{
position:absolute;
top:50%;
left:50%;
}
html
<body>
<div id="aside">
<div id="hole"></div>
</div>
</body>
效果
