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Recently I interviewed Stefan Judis for my upcoming book. We discussed CSS custom properties, and something interesting happened.

We had a period of a few minutes where we were talking past one another, because, as it turns out, we have completely opposite ideas about the use of CSS custom properties. I had never considered his approach, and I found it interesting enough to write this quick post.

Option 1

Take several site components, each with their own link and hover/focus colours. We want to use custom properties for those colours. Exactly how do we do that?

Before my discussion with Stefan that wasn’t even a question for me. I would do this:

.component1 {
	--linkcolor: red;
	--hovercolor: blue;
}

.component2 {
	--linkcolor: purple;
	--hovercolor: cyan;
}

a {
	color: var(--linkcolor);
}

a:hover,a:focus {
	color: var(--hovercolor)
}

I set the normal and hover/focus colour as a custom property, and leave the definition of those properties to the component the link appears in. The first and second component each define different colours, which are deployed in the correct syntax. Everything works and all’s well with the world.

As far as I can see now this is the default way of using CSS custom properties. I wasn’t even aware that another possibility existed.

Option 2

Stefan surprised me by doing almost the complete opposite. He uses only a single variable and changes its value where necessary:

.component1 {
	--componentcolor: red;
}

.component1 :is(a:hover,a:focus) {
	--componentcolor: blue;
}
	
.component2 {
	--componentcolor: purple;
}

.component2 :is(a:hover,a:focus) {
	--componentcolor: cyan;
}
	
a {
	color: var(--componentcolor)		
}

At first I was confused. Why would you do this? What’s the added value of the custom property? Couldn’t you just have entered the colour values in the component styles without using custom properties at all?

Well, yes, you could. But that’s not Stefan’s point.

The point

In practice, component definitions have way more styles than just colours. There’s a bunch of box-model properties, maybe a display, and possibly text styling instructions. In any case, a lot of lines of CSS.

If you use custom properties only for those CSS properties that will change you give future CSS developers a much better and quicker insight in how your component works. If the definition uses a custom property that means the property may change in some circumstances. If it uses a fixed definition you know it’s a constant.

Suppose you encounter this component definition in a codebase you just inherited:

.component {
	--color: red;
	--background: blue
	--layout: flex;
	--padding: 1em;
	--borderWidth: 0.3em;
	display: var(--layout);
	color: var(--color);
	background: var(--background);
	padding: var(--padding);
	border: var(--borderWidth) solid black;
	margin: 10px;
	border-radius: 2em;
	grid-template-columns: repeat(3,1fr);
	flex-wrap: wrap;
}

Now you essentially found a definition file. Not only do you see the component’s default styles, you also see what might change and what will not. For instance, because the margin and border-radius are hard-coded you know they are never changed. In the case of the border, only the width changes, not the style or the colour. Most other properties can change.

The use of display: var(--layout) is particularly revealing. Apparently something somewhere changes the component’s layout from grid to flexbox. Also, if it’s a grid it has three equal columns, while if it’s a flexbox it allows wrapping. This suggests that the flexbox layout is used on narrower screens, switching to a grid layout on wider screens.

Where does the flexbox change to a grid? As a newbie to this codebase you don’t know, but you can simply search for --layout: grid and you’ll find it, probably neatly tucked away in a media query somewhere. Maybe there is a basic layout as well, which uses neither flexbox nor grid? Search for --layout: block and you’ll know.

Thus, this way of using custom properties is excellently suited for making readable code bases that you can turn over to other CSS developers. They immediately know what changes and what doesn’t.

Teaching aid?

There’s another potential benefit as well: this way of using custom properties, which are essentially variables, aligns much more with JavaScript’s use of variables. You set an important variable at the start of your code, and change it later on if necessary. This is what you do in JavaScript all the time.

Picking an option

Which option should you pick? That’s partly a matter of personal preference. Since the second option is still fairly new to me, and I rarely work on large projects, I am still feeling my way around it. Right at this moment I prefer the first way because I’m used to it. But that might change, given some extra time.

Still, I think Stefan is on to something. I think that his option is very useful in large codebases that can be inherited by other developers. I think it deserves careful consideration.

I’m writing the position: sticky part of my book, and since I never worked with sticky before I’m not totally sure if what I’m saying is correct.

This is made worse by the fact that there are no very clear tutorials on sticky. That’s partly because it works pretty intuitively in most cases, and partly because the details can be complicated.

So here’s my draft 1 of position: sticky. There will be something wrong with it; please correct me where needed.

The inset properties are top, right, bottom and left. (I already introduced this terminology earlier in the chapter.)

Introduction

position: sticky is a mix of relative and fixed. A sticky box takes its normal position in the flow, as if it had position: relative, but if that position scrolls out of view the sticky box remains in a position defined by its inset properties, as if it has position: fixed. A sticky box never escapes its container, though. If the container start or end scrolls past the sticky box abandons its fixed position and sticks to the top or the bottom of its container.

It is typically used to make sure that headers remain in view no matter how the user scrolls. It is also useful for tables on narrow screens: you can keep headers or the leftmost table cells in view while the user scrolls.

Scroll box and container

A sticky box needs a scroll box: a box that is able to scroll. By default this is the browser window — or, more correctly, the layout viewport — but you can define another scroll box by setting overflow on the desired element. The sticky box takes the first ancestor that could scroll as its scroll box and calculates all its coordinates relative to it.

A sticky box needs at least one inset property. These properties contain vital instructions, and if the sticky box doesn’t receive them it doesn’t know what to do.

A sticky box may also have a container: a regular HTML element that contains the sticky box. The sticky box will never be positioned outside this container, which thus serves as a constraint.

The first example shows this set-up. The sticky <h2> is in a perfectly normal <div>, its container, and that container is in a <section> that is the scroll box because it has overflow: auto. The sticky box has an inset property to provide instructions. The relevant styles are:

section.scroll-container {
	border: 1px solid black;
	width: 300px;
	height: 300px;
	overflow: auto;
	padding: 1em;
}

div.container {
	border: 1px solid black;
	padding: 1em;
}

section.scroll-container h2 {
	position: sticky;
	top: 0;
}

The rules

Now let’s see exactly what’s going on.

A sticky box never escapes its containing box. If it cannot obey the rules that follow without escaping from its container, it instead remains at the edge. Scroll down until the container disappears to see this in action.

A sticky box starts in its natural position in the flow, as if it has position: relative. It thus participates in the default flow: if it becomes higher it pushes the paragraphs below it downwards, just like any other regular HTML element. Also, the space it takes in the normal flow is kept open, even if it is currently in fixed position. Scroll down a little bit to see this in action: an empty space is kept open for the header.

A sticky box compares two positions: its natural position in the flow and its fixed position according to its inset properties. It does so in the coordinate frame of its scroll box. That is, any given coordinate such as top: 20px, as well as its default coordinates, is resolved against the content box of the scroll box. (In other words, the scroll box’s padding also constrains the sticky box; it will never move up into that padding.)

A sticky box with top takes the higher value of its top and its natural position in the flow, and positions its top border at that value. Scroll down slowly to see this in action: the sticky box starts at its natural position (let’s call it 20px), which is higher than its defined top (0). Thus it rests at its position in the natural flow. Scrolling up a few pixels doesn’t change this, but once its natural position becomes less than 0, the sticky box switches to a fixed layout and stays at that position.

It does the same for bottom, but remember that a bottom is calculated relative to the scroll box’s bottom, and not its top. Thus, a larger bottom coordinate means the box is positioned more to the top. Now the sticky box compares its default bottom with the defined bottom and uses the higher value to position its bottom border, just as before.

With left, it uses the higher value of its natural position and to position its left border; with right, it does the same for its right border, bearing in mind once more that a higher right value positions the box more to the left.

If any of these steps would position the sticky box outside its containing box it takes the position that just barely keeps it within its containing box.

Details

The four inset properties act independently of one another. For instance the following box will calculate the position of its top and left edge independently. They can be relative or fixed, depending on how the user scrolls.

p.testbox {
	position: sticky;
	top: 0;
	left: 0;
}

Setting both a top and a bottom, or both a left and a right, gives the sticky box a bandwidth to move in. It will always attempt to obey all the rules described above. So the following box will vary between 0 from the top of the screen to 0 from the bottom, taking its default position in the flow between these two positions.

p.testbox {
	position: sticky;
	top: 0;
	bottom: 0;
}

No container

So far we put the sticky box in a container separate from the scroll box. But that’s not necessary. You can also make the scroll box itself the container if you wish. The sticky element is still positioned with respect to the scroll box (which is now also its container) and everything works fine.

Several containers

Or the sticky item can be several containers removed from its scroll box. That’s fine as well; the positions are still calculated relative to the scroll box, and the sticky box will never leave its innermost container.

Changing the scroll box

One feature that catches many people (including me) unaware is giving the container an overflow: auto or hidden. All of a sudden it seems the sticky header doesn’t work any more.

What’s going on here? An overflow value of auto, hidden, or scroll makes an element into a scroll box. So now the sticky box’s scroll box is no longer the outer element, but the inner one, since that is now the closest ancestor that is able to scroll.

The sticky box appears to be static, but it isn’t. The crux here is that the scroll box could scroll, thanks to its overflow value, but doesn’t actually do so because we didn’t give it a height, and therefore it stretches up to accomodate all of its contents.

Thus we have a non-scrolling scroll box, and that is the root cause of our problems.

As before, the sticky box calculates its position by comparing its natural position relative to its scroll box with the one given by its inset properties. Point is: the sticky box doesn’t scroll relative to its scroll box, so its position always remains the same. Where in earlier examples the position of the sticky element relative to the scroll box changed when we scrolled, it no longer does so, because the scroll box doesn’t scroll. Thus there is no reason for it to switch to fixed positioning, and it stays where it is relative to its scroll box.

The fact that the scroll box itself scrolls upward is irrelevant; this doesn’t influence the sticky box in the slightest.

One solution is to give the new scroll box a height that is too little for its contents. Now the scroll box generates a scrollbar and becomes a scrolling scroll box. When we scroll it the position of the sticky box relative to its scroll box changes once more, and it switches from fixed to relative or vice versa as required.

Minor items

Finally a few minor items:

• It is no longer necessary to use position: -webkit-sticky. All modern browsers support regular position: sticky. (But if you need to cater to a few older browsers, retaining the double syntax doesn’t hurt.)
• Chrome (Mac) does weird things to the borders of the sticky items in these examples. I don’t know what’s going on and am not going to investigate.

Last week Krijn and I decided to cancel performance.now() 2021. Although it was the right decision it leaves me in financially fairly dire straits. So I’m looking for new jobs and/or donations.

Even though the Corona trends in NL look good, and we could probably have brought 350 people together in November, we cannot be certain: there might be a new flare-up. More serious is the fact that it’s very hard to figure out how to apply the Corona checks Dutch government requires, especially for non-EU citizens. We couldn’t figure out how UK and US people should be tested, and for us that was the straw that broke the camel’s back. Cancelling the conference relieved us of a lot of stress.

Still, it also relieved me of a lot of money. This is the fourth conference in a row we cannot run, and I have burned through all my reserves. That’s why I thought I’d ask for help.

So ...

Has QuirksMode.org ever saved you a lot of time on a project? Did it advance your career? If so, now would be a great time to make a donation to show your appreciation.

I am trying my hand at CSS coaching. Though I had only few clients so far I found that I like it and would like to do it more. As an added bonus, because I’m still writing my CSS for JavaScripters book I currently have most of the CSS layout modules in my head and can explain them straight away — even stacking contexts.

Or if there’s any job you know of that requires a technical documentation writer with a solid knowledge of web technologies and the browser market, drop me a line. I’m interested.

Anyway, thanks for listening.

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