背景半透明(HTML、CSS)

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>背景半透明</title>
    <style>
        div {
            width: 300px;
            height: 300px;
            /*       background-color:black; */
            /*  background-color: rgba(0, 0, 0, 0.3); */
            background-color: rgba(0, 0, 0, .1);

        }