Posts from April 2022

Flexibly Centering an Element with Side-Aligned Content

Published 2 years, 6 months past

In a recent side project that I hope will become public fairly soon, I needed to center a left-aligned list of links inside the sides of the viewport, but also line-wrap in cases where the lines got too long (as in mobile). There are a few ways to do this, but I came up with one that was new to me. Here’s how it works.

First, let’s have a list.  Pretend each list item contains a link so that I don’t have to add in all the extra markup.

<ol>
	<li>Foreword</li>
	<li>Chapter 1: The Day I Was Born</li>
	<li>Chapter 2: Childhood</li>
	<li>Chapter 3: Teachers I Admired</li>
	<li>Chapter 4: Teenage Dreaming</li>
	<li>Chapter 5: Look Out World</li>
	<li>Chapter 6: The World Strikes Back</li>
	<li>Chapter 7: Righting My Ship</li>
	<li>Chapter 8: In Hindsight</li>
	<li>Afterword</li>
</ol>

Great. Now I want it to be centered in the viewport, without centering the text. In other words, the text should all be left-aligned, but the element containing them should be as centered as possible.

One way to do this is to wrap the <ol> element in another element like a <div> and then use flexbox:

div.toc {
	display: flex;
	justify-content: center;
}

That makes sense if you want to also vertically center the list (with align-items: center) and if you’re already going to be wrapping the list with something that should be flexed, but neither really applied in this case, and I didn’t want to add a wrapper element that had no other purpose except centering. It’s 2022, there ought to be another way, right? Right. And this is it:

ol {
	max-inline-size: max-content;
	margin-inline: auto;
}

I also could have used width there in place of max-inline-size since this is in English, so the inline axis is horizontal, but as Jeremy pointed out, it’s a weird clash to have a physical property (width) and a logical property (margin-inline) working together. So here, I’m going all-logical, which is probably better for the ongoing work of retraining myself to instinctively think in logical directions anyway.

Thanks to max-inline-size: max-content, the list can’t get any wider (more correctly: any longer along the inline axis) than the longest list item. If the container is wider than that, then margin-inline: auto means the ol element’s box will be centered in the container, as happens with any block box where the width is set to a specific amount, there’s leftover space in the container, and the side margins of the box are set to auto. This is as if I’d pre-calculated the maximum content size to be (say) 434 pixels wide and then declared max-inline-size: 434px.

The great thing here is that I don’t have to do that pre-calculation, which would be very fragile in any case. I can just use max-content instead. And then, if the container ever gets too small to fit the longest bit of content, because the ol was set to max-inline-size instead of just straight inline-size, it can fill out the container as block boxes usually do, and the content inside it can wrap to multiple lines.

Perhaps it’s not the most common of layout needs, but if you find yourself wanting a lightweight way to center the box of an element with side-aligned content, maybe this will work for you.

What’s nice about this is that it’s one of those simple things that was difficult-to-impossible for so long, with hacks and workarounds needed to make it work at all, and now it… just works.  No extra markup, not even any calc()-ing, just a couple of lines that say exactly what they do, and are what you want them to do.  It’s a nice little example of the quiet revolution that’s been happening in CSS of late.  Hard things are becoming easy, and more than easy, simple.  Simple in the sense of “direct and not complex”, not in the sense of “obvious and basic”.  There’s a sense of growing maturity in the language, and I’m really happy to see it.

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