@supports

dabblet.css

/*
 *	@supports
 */

#supportsCSSVariables {  
	display: none;
	font-size: 70px;
	font-family: monospace;
}

@supports (--css: variables){  
	#supportsCSSVariables {    
		display: block;  
	}
}

dabblet.html

<div id="supportsCSSVariables">  
	This browser supports CSS variables.
</div>

dabblet.js

// alert('Hello world!');

settings.json

{"view":"separate","fontsize":"100","seethrough":"","prefixfree":"1","page":"all"}