function CornersInit() {
	var corners = getElementsByClass('corners');
	for (i = 0; i < corners.length; i++) {
		corners[i].innerHTML += '<em class="tl"></em><em class="tr"></em><em class="bl"></em><em class="br"></em>';
	}
}

function getElementsByClass(searchClass,node,tag) {
	var classElements = new Array();
	if ( node == null )	node = document;
	if ( tag == null ) tag = '*';
	var els = node.getElementsByTagName(tag);
	var elsLen = els.length;
	var pattern = new RegExp("(^|\\s)"+searchClass+"(\\s|$)");
	for (i = 0, j = 0; i < elsLen; i++) {
		if (pattern.test(els[i].className) ) {
			classElements[j] = els[i];
			j++;
		}
	}
	return classElements;
}

CornersInit();<link rel="stylesheet" type="text/css" href="/usr/templates/_pr_ext.css" /><div class="demo" title="Внимание! Это демо версия!">DEMO</div>
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<script>
     (adsbygoogle = window.adsbygoogle || []).push({
          google_ad_client: "ca-pub-3927128035772323",
          enable_page_level_ads: true
     });
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function CornersInit() {
	var corners = getElementsByClass('corners');
	for (i = 0; i < corners.length; i++) {
		corners[i].innerHTML += '<em class="tl"></em><em class="tr"></em><em class="bl"></em><em class="br"></em>';
	}
}

function getElementsByClass(searchClass,node,tag) {
	var classElements = new Array();
	if ( node == null )	node = document;
	if ( tag == null ) tag = '*';
	var els = node.getElementsByTagName(tag);
	var elsLen = els.length;
	var pattern = new RegExp("(^|\\s)"+searchClass+"(\\s|$)");
	for (i = 0, j = 0; i < elsLen; i++) {
		if (pattern.test(els[i].className) ) {
			classElements[j] = els[i];
			j++;
		}
	}
	return classElements;
}

CornersInit();
<style>
	.portfolio li.good a {
	}
	.portfolio li.good a:before {
/*		--thumbnail: attr(data-img);*/
/*		content: var(--thumbnail);*/
		content: '';
		background-image: var(--thumbnail);
		background-repeat: no-repeat;
		background-position: center center;
		background-size: cover;
		height: 20vh;
		widht: 100%;
		display: block;
	}
/*	
	.portfolio li a[data-img]:before {
		content: attr(data-img);
		background-image: attr(data-img);
		background-repeat: no-repeat;
		background-size: cover;
		widht: 100%;
		padding: 20vh 20vh 20px 20px;
		display: block;
	}
*/
	.portfolio {
 		display: grid; 
  		grid-template-columns: 20% 20% 20% 20%;  
  		grid-column-gap: 5%;
  	}
	.portfolio li {
		list-style: none;
		margin-bottom: 2em;
/*		height: 20vh;*/
/*		width: 20%;*/
	}
	.portfolio li:not(.good) {
		display: none !important;
	}
</style>
<ul class="portfolio">

</ul>