One line of css code realizes the gray mourning color of the website

During the National Day of Mourning, all Internet websites turned gray overnight. So how is this achieved? It can basically be achieved with one line of CSS code.
In fact, using the filter property of css3 defilter

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;
}

The effect is as follows: