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HTML 

              
                <main>

	<h1>How <em>not</em> to use percentages for height in a flex container</h1>

	<div class="flex-container">
		<div class="column-1">
			<figure class="killed-by-percentages"></figure>
			<h2 class="dead">🤔 Wait a sec, what am I doing out here?</h2>
		</div>
		<div class="column-2">
			<ul class="list-of-things">
				<li>Thing #1</li>
				<li>Thing #2</li>
				<li>Thing #3</li>
			</ul>
		</div>
	</div>

	<div class="we-got-problems">
		<p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Consequuntur possimus temporibus, totam repellat molestias voluptatum placeat voluptates dicta aspernatur, quos quia corrupti, atque architecto magni eos doloribus soluta vero hic.</p>
		<p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Consequuntur possimus temporibus, totam repellat molestias voluptatum placeat voluptates dicta aspernatur, quos quia corrupti, atque architecto magni eos doloribus soluta vero hic.</p>
		<p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Consequuntur possimus temporibus, totam repellat molestias voluptatum placeat voluptates dicta aspernatur, quos quia corrupti, atque architecto magni eos doloribus soluta vero hic.</p>
	</div>

	<section class="til">
		<h2>Today I Learned</h2>
		<p>As soon as you assign a percentage-based height inside a flex child (literally anything inside that affects that child’s height, even grandchild elements), if any element uses a percentage-based height then that height is removed from the parent height calculation.</p>
		<p>The percentage ignores the current column’s height and sets 100% = the next tallest column.</p>
		<p>And then you sob because the rest of your content got pushed outside the container.</p>
	</section>

</main>
!

CSS 

              
                .flex-container {
	display: flex;
}

.column-1 {
	width: 60%;
}

.column-2 {
	width: 40%;
}

.list-of-things li {
	margin-bottom: 1rem;
	padding: 1rem;
	background: lightgray;
}

.killed-by-percentages {
	height: 100%;
	max-height: 200px;
	background: url('https://i.kym-cdn.com/entries/icons/mobile/000/018/012/this_is_fine.jpg') center top / cover no-repeat;
}



/* ------------------------------ */
/* Demo styles                    */
/* ------------------------------ */

body {
	background: tomato;
}

main {
	max-width: 100ch;
	margin: auto;
}

div {
	outline: 1px dotted black;
}

figure {
	margin: 0;
}

.flex-container {
	background: white;
}

.killed-by-percentages {	
	outline: 1px dashed red;
}

.dead {
	background: yellow;
	font-style: italic;
}

.we-got-problems {
	position: relative;
	z-index: -1;
}

.til {
	max-width: 75ch;
	margin: auto;
}
!

JS
!
999px

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