<div class="cover"></div>

<style> .cover{     position:fixed;     top: 0px;     left: 0px;     outline:5000px solid rgba(0, 0, 0, 0.3);     z-index: 99999; } </style>

<script> var brightness; //显示遮罩 function cover(brightness) {     if (typeof(div) == 'undefined') {         div = document.createElement('div');         div.setAttribute('style', 'position:fixed;top:0;left:0;outline:5000px solid;z-index:99999;');         document.body.appendChild(div);     } else {         div.style.display = '';     }     div.style.outlineColor = 'rgba(0,0,0,' + brightness + ')'; } //事件监听 window.addEventListener('keydown', function(e) {     if (e.altKey && e.keyCode == 90) { //Alt+Z:打开夜间模式         cover(brightness = 0.3);     }     if (e.altKey && e.keyCode == 88) { //Alt+X:关闭         cover(brightness = 0);     }     if (e.altKey && e.keyCode == 38) { //Alt+↑:增加亮度         if (brightness - 0.05 > 0.05) cover(brightness -= 0.05);     }     if (e.altKey && e.keyCode == 40) { //Alt+↓:降低亮度         if (brightness + 0.05 < 0.95) cover(brightness += 0.05);     } }, false); </script>