一、HTML方式

<style type="text/css">
  html{
    -webkit-filter: grayscale(100%);
    -moz-filter: grayscale(100%);
    -ms-filter: grayscale(100%);
    -o-filter: grayscale(100%);
    filter:progid:DXImageTransform.Microsoft.BasicImage(grayscale=1);
    _filter:none;
}
</style>

二、CSS方式

html {
  -webkit-filter: grayscale(100%);
  -moz-filter: grayscale(100%);
  -ms-filter: grayscale(100%);
  -o-filter: grayscale(100%);
  filter:progid:DXImageTransform.Microsoft.BasicImage(grayscale=1);
  _filter:none;
}

PS:蜡烛Emoji - 🕯️