Auto column width grid

HTML

<div class=cont><div class=cell>cell</div></div>

CSS

.cont {
	display: grid;
	grid-template-columns: repeat(auto-fill, minmax(10em, 1fr));
	width: 500px;
	border: solid 3px cyan;
}
.cell {
	background: magenta;
	border: solid 1px;
}

JavaScript

window.onclick = function() {
	document.querySelector('.cont').insertAdjacentHTML('beforeend', '<div class=cell>celkgugkftfkjhvktfitktfgkjftfitkgfgfyturtdujdyyfiytl</div>');
};