全页面覆盖

<div 
  style="position:fixed;top:0;left:0;right:0;bottom:0;z-index:9999999999;background:#000;font-size:50px;color:#fff">
<span>安全要加强，大佬说的对</span>
</div>

全屏

function isFullscreen() {
  return document.fullscreenElement || document.mozFullScreenElement
    || document.webkitFullscreenElement || document.msFullscreenElement;
}
const fullscreenBtn = document.getElementById('fullscreen-btn');
fullscreenBtn.addEventListener('click', e => {
  fullscreenBtn.blur();
  if (isFullscreen()) {
    if (document.exitFullscreen) document.exitFullscreen();
    else if (document.msExitFullscreen) document.msExitFullscreen();
    else if (document.mozCancelFullScreen) document.mozCancelFullScreen();
    else if (document.webkitExitFullscreen) document.webkitExitFullscreen();
  } else {
    if (document.body.requestFullscreen) document.body.requestFullscreen();
    else if (document.body.msRequestFullscreen) document.body.msRequestFullscreen();
    else if (document.body.mozRequestFullScreen) document.body.mozRequestFullScreen();
    else if (document.body.webkitRequestFullscreen) document.body.webkitRequestFullscreen(Element.ALLOW_KEYBOARD_INPUT);
  }
});

你不知道的HTML CSS

原文：https://www.cnblogs.com/develon/p/12260385.html