JSFiddle - React, Tailwind, and code Playground

HTML

<a href="#login-box" class="log-in-link">ссылка</a>
<p>Тест на прозрачность</p>

CSS

#mask {
				display: none;
				background: #000; 
				position: fixed; left: 0; top: 0; 
				width: 100%; 
				height: 100%;
				opacity: 0.8;
				z-index: 999;
			}

JavaScript

(function() {

	$('a.log-in-link').on('click', function() {
		
		$('body').append('<div id="mask"></div>');
		$('#mask').fadeIn(300);
		
		return false;
	});
    
    $(document).on("click", "#mask", function() {
        $('#mask').fadeOut(300, function() {
		$('#mask').remove();  
		}); 
		
		return false;
    }); 
    
})();