simple css

gistfile1.txt

* {
	box-sizing: border-box;
}
ul {
	padding: 0;
	margin: 0;
}
body {
	background-color: #eee;
	margin: 0px;
	padding: 0px;
	font-size: 1.5rem;
	line-height: 2rem;
}
iframe,img {
	max-width: 100%;
}
div {
	border: 1px solid silver;
}
.row {
	display: flex;
	flex-wrap: wrap;
	align-items: flex-start;
	justify-content: center;
	align-content: flex-start;
	margin: auto;
}
.row.sized {
	max-width: 50rem;
}
.row > *.full {
	width: 100%;
	max-width: 100%;
}

.content {
	display: flex;
	flex-direction: column;
	justify-content: flex-start;
	align-items: center;
}
.content > * {
	max-width: 100%;
	width: 50rem;
}
.content > *.full {
	width: 100%;
	max-width: 100%;
}
.content > *.fill {
	max-width: 100%;
}

@media screen and (max-width: 50rem) {
	.content > * {
		max-width: 100%;
		/* overwrite width 50rem  */
		width: 100%;
	}
}