README.md

Explainer: scroll-start-target

Introduction

This is an explainer for scroll-start-target: a CSS property that gives web authors the ability to set the initial scroll offset of a scrollable container by indicating content that the container should be initially scrolled to.

Background

Web authors often wish to have scrollable containers scrolled away from the default (0,0) position when the container first appears on screen. This could be, for example, to reflect some state of the page which might make some of the scrollable container's content more relevant than other content within the container. In some other cases, websites would like to present users with content first, and then have them scroll up to reveal an item, e.g. a search bar like in iOS's Messages app, and in Spotify.

On the web, one way to achieve this is via fragment identifiers in anchor links, but this method has the constraint of being able to target only one item.

Another approach is to create a temporary CSS animation which briefly modifies the scroll-snap-align property of the target item so as to cause a scroll to it:

.container {
  ...
  scroll-snap-type: x mandatory;
}

.target {
  ...
  animation: --snap 0.01s linear;
}

@keyframes --snap {
	from {
		scroll-snap-align: start;	
	}
}

This approach is rather brittle and would not work well in a scenario where the target is to remain in view even through layout changes occurring as the page loads.

Another way is to use one of JavaScript's scrolling APIs, e.g. scrollIntoView, to bring the relevant content into view after the page has been loaded. However, using JavaScript to achieve the desired style of a page is often less than ideal as it can result in degraded performance and can be prone to error. For example, similar to scroll anchoring, web authors would need to account for the dynamic way in which content is fetched and loaded into a page such that changes in the layout of the page are necessary. This means that: to achieve the intended effect of scroll-start-target, it may not be enough to write:

window.onload = () => {
  ...
  const scroll_target = document.getElementById("scroll-target");
  scroll_target.scrollIntoView();
  ...
}

as the window load event will only fire once. The author will need to:

• detect the occurrence of layout changes that result in their desired element not being scrolled into view, and
• invoke scrollIntoView again.

If using scrollIntoView to keep a particular element in view, the author would also need to be mindful of interfering with a scrolling gesture from their user i.e. if the user has scrolled away from the targeted content, scroll-start-target no longer keeps the scroll container anchored to the content.

Additionally, the scrollable container in question may not appear on the page until after the load event, e.g. until a user clicks a "Show Gallery" button like in the gallery example below.

Proposal

Introduce a CSS property called scroll-start-target which can be used to indicate that the scroll container of the affected element (the target) should, on its initial appearance/layout in the page, be scrolled to the target.

After the user agent detects an "explicit" scroll on the scroll container, scroll-start-target isn't active anymore and the scroll container can be freely scrolled. An explicit scroll would be any scroll arising from a user's gesture, or a programmatic scroll API.

scroll-start-target is also not to interfere with scrolling due to a fragment identifiers in a URL.

Use Cases

• an image gallery that ought to start with the middle image in view,
• a page in which the author would like to first present content to the user and then have them scroll upwards to reveal a searchbar,
• a menu/list of options which starts scrolled to a user's previously-selected choices,
• a menu/list of options among which the author predicts some to be more relevant to the user than others due, perhaps, to prior user activity,
• a long page where the author wants to save the user the effort of scrolling to a particular section.

Examples

User Settings

In this example when the "Maintenance Page" is loaded, the scroll container, when first displayed, appears scrolled to the section corresponding to the selected drone.

Gallery

In this example the image gallery is not displayed until the "Show Gallery" button is clicked after which the gallery is revealed and is scrolled to the middle image.

Syntax

The scroll-start-target property takes one parameter whose only valid values are:

• auto, indicating the element should be scrolled to, and
• none, the default value, indicating the element isn't to be scrolled to.

<style>
.scrollcontainer {
    overflow-x: scroll;
}
.target {
    scroll-start-target: [auto|none];
}
</style>
...
<div class="scrollcontainer">
    <div></div>
    <div></div>
    <div class="target"></div>
    <div></div>
    <div></div>
</div>
...

Alternatives & Other Considerations

Per-axis Parameters

One alternative option for scroll-start-target's syntax would allow specifying values for the block and inline axes separately:

<style>
...
div {
    scroll-start-target: <block> <inline>;
}
...
</style>

where <block> and <inline> could independently be auto or none. By setting the bias on the block and inline axis independently of each other, this syntax would make it possible for separate targets to be scrolled to in the different axes such that neither target ends up within the scroll container's scroll port which runs contrary to the purpose of scroll-start-target.

Competing scroll-start-targets

It is possible for more than one element within the content of a scrollable container to have its scroll-start-target set to auto, meaning the scrollable container has to choose which scroll-start-target to scroll to. Overall, user agents are to handle such scenarios by giving priority to the element which comes first in DOM order.

Competing scroll-start-targets & DOM-order

While priority is to be given to the scroll-start-target which comes first in DOM order, there is some consideration for a scenario in which the author desires a best effort attempt at bringing all the specified scroll-start-targets into view. Specifically, where mutliple scroll-start-targets exist, user agents are to scroll to each target in reverse-DOM-order, i.e. starting from the last and working their way up to the first. At the moment, this is effectively the same as only scrolling to the first in DOM order. However, these steps are chosen in a way that will work well with a potential future css property that allow a nearest scroll alignment similar to scrollIntoView's nearest argument so that the reverse-DOM order procedure can be closer to "best effort."

As an example of a situation where a developer might want multiple scroll-start-targets consider this demo where the "done" items in the TODO list are all scroll-start-targets. When the user dismisses a done item, the next one, by virtue of being a scroll-start-target, comes into view, making the process of dismissing all done items very convenient for the user.