HTML-and-CSS-only collapsible

While making this website, I decided to challenge myself and reduce to the absolute minimum the usage of Javascript. This implied that the navigation on this website had to work without Javascript.

Why?

Well, Javascript is a powerful tool! It can store data and do some processing entirely client-side, which is really useful for some client-side web applications.
There's some issues though, some of its applications can be questionable as some will use it to do some tracking on users, which is why some people choose to disable it. It also requires more processing than pure HTML & CSS. Since, HTML 5 and CSS 3 got released, with a ton of new features like animations, better selectors, and more stuff. A lot of websites continue to use Javascript while not necessarily having a need for it it in every parts.

To come back the the navigation part: this website uses a side navigation, on desktop it's fine. Though, on mobile it has some issues, it needs to be hidden away (collapsed).

HTML 5 offers us the element <details>, which allows to hide stuff away with a trigger to open and see its content. You can specify a summary, which is always displayed and acts as a trigger. The main issue with it is it only allows one and only one trigger, in the case of this website's side navigation it needs at least 2 triggers.

But there's another trick, which I found the solution after visiting Wikipedia, which I am describing here.

1. The theory
2. The HTML
3. The CSS
4. Result
5. Going further
   1. Animation
      1. Animated Result
   2. Sidenav
      1. Sidenav Result
6. Conclusion

The theory

Let's look into some HTML components and CSS selectors.

HTML checkboxes have some specific behaviors: when a user clicks on it, it gets checked and vice-versa. And we can check if a checkbox is checked with a CSS selector :checked.

The other specific behavior comes from labels which are linked to a checkbox, when clicking a <label> targetting a checkbox, it will act as if the checkbox was clicked.

And last, we can select neighbor elements in CSS with ~.

I think you might see where I'm going with this.

If you don't: the goal is to use a hidden checkbox to store the open/close state, and labels for the triggers.

The HTML

Let's see how we can do it in HTML, first we can enclose the entire thing in a <div> to simplify it.
Then we will have 3 elements: the hidden checkbox, the content to display, and a trigger.

<div class="collapsible_wrapper">
	<input type="checkbox" id="first_collapsible_trigger" class="collapsible_internal_trigger"
		aria-hidden="true">

	<label for="first_collapsible_trigger" class="collapsible_trigger">
		Click here!
	</label>

	<div class="collapsible_content">
		<p>Hello world!</p>
	</div>
</div>

The CSS

Now that we have the HTML, we can focus on the stylesheet.
The goal is to always hide the checkbox, and to hide the content depending on the state of the checkbox.

/* Avoid weird spacing. */
.collapsible_wrapper {
	display: flex;
	flex-direction: column;
}

/* We hide the checkbox. */
.collapsible_internal_trigger {
	display: none;
}

.collapsible_trigger {
	cursor: pointer;
	color: crimson;
}

/* We hide the content by default. */
.collapsible_content {
	visibility: collapse;
	height: 0;
}

/* When the collapsible is opened, display the content. */
.collapsible_internal_trigger:checked ~ .collapsible_content {
	visibility: visible;
	height: 100%;
}

Result

Going further

Animation

Ok, maybe we want to animate now, let's take the same HTML but with _anim appended to the contant class, and a different CSS stylesheet:

/* We hide the content by default. */
.collapsible_content_anim {
	visibility: collapse;
	height: 0;
	transform: translateX(-100%);
	opacity: 0;
	transition: transform 500ms ease, opacity 500ms ease;
}

/* When the collapsible is opened, display the content. */
.collapsible_internal_trigger:checked ~ .collapsible_content_anim {
	visibility: visible;
	height: 100%;
	transform: translateX(0%);
	opacity: 100%;
}

/* Remove animations if the user prefers reduced motion. */
@media (prefers-reduced-motion) {
	.collapsible_content_anim {
		transition: none;
	}
}

Warning: you cannot use dimension transitions with percentage values for some reason. It's one of the very frustrating things as using absolute values is quite bad.

Animated Result

Sidenav

Now, let's see how I did my side navigation.

First, instead of having only one trigger there is 2 of them: one to open and one to close. Then the content takes a whole side of the screen.

<div class="ls_sidenav_wrapper">
	<input type="checkbox" id="example_nav_trigger" class="ls_sidenav_internal_trigger" 
		aria-hidden="true">
	<div id="example_nav" class="example_sidenav">
		Hello world!
		<p>
			To close this sidenav, please click on the darkened area!
		</p>
	</div>
	<label for="example_nav_trigger" class="ls_btn" ls_variant="fab" aria-role="menu" aria-label="Menu" aria-description="Open the example navigation menu.">
		<svg width="40" height="40" viewBox="0 0 40 40" stroke="var(--ls_theme_on_primary)" stroke-width="2px" shape-rendering="crispedges">
			<line x1="12" y1="14" x2="28" y2="14"></line>
			<line x1="12" y1="20" x2="28" y2="20"></line>
			<line x1="12" y1="26" x2="28" y2="26"></line>
		</svg>
	</label>
	<label for="example_nav_trigger" class="example_sidenav_darkened"></label>
</div>

.example_sidenav {
	/* We position the sidenav on the right, in a way so it takes the entire height. */
	position: fixed;
	top: 0;
	right: 0;
	margin: 0;
	height: 100%;
	max-width: 260px;
	min-width: 260px;

	background-color: var(--ls_theme_background_accentuated);
	overflow-y: auto;

	padding: 2px 4px;

	z-index: 128; /* We want the sidenav to be on top. */

	visibility: hidden;
	transform: translateX(+150%); /* Would be -150% if the sidenav was on the left. */

	transition: transform 500ms ease, visibility 500ms ease;
}

/* The darkened area, it covers the entire screen, is under the sidenav area */
.example_sidenav_darkened {
	visibility: hidden;
	opacity: 0;
	position: fixed;
	background-color: rgba(0, 0, 0, 0.4);
	cursor: pointer;
	z-index: 64;

	top: 0;
	bottom: 0;
	right: 0;
	left: 0;
}

.ls_sidenav_internal_trigger:checked ~ .example_sidenav {
	visibility: visible;
	transform: translateX(0%);
}

.ls_sidenav_internal_trigger:checked ~ .example_sidenav_darkened {
	visibility: visible;
	opacity: 100%;
	transition: opacity 500ms ease, visibility 500ms ease;
	transform: translateX(0%);
}

/* Remove animations if the user prefers reduced motion. */
@media (prefers-reduced-motion) {
	.example_sidenav {
		transition: none;
	}
}

Sidenav Result

And voilà!

Here's a collapsible side navigation which opens from the right:

Conclusion

Now you too have the knowledge of Javascript-less collapsible containers!

I hope it will help you some way, but there are more things to note:

As mentioned in the introduction, the <details> HTML element can replace some of the basic examples I've shown, it has the same purpose and fulfills it better semantically. This means stuff like screen readers will support it much better.
For those who don't know, the way it works is you define a <details> container, with a <summary> child element, which contains elements that are always displayed and serves as a trigger, the other child elements are the content.

For example:

<details>
	<summary>Click me!</summary>
	Hello world!
</details>

So, the use case of this method would more be for dropdown menus, and side navigation that needs to be hidden on mobile.

There's also some limitations, for example making an animation changing the height of the content to display it smoothly is not possible unless the height properties are hardcoded to specific values (no percentages!), it's quite awful to do.

If you do nesting, hardcoding those values can become quite an issue.

It's important to know limitations of the technologies we manipulate, despite those this trick is still pretty useful.