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Parallax

This document is an explainer for a potential future web platform feature enabling parallax elements.

Sample code

<parallax-container>
  <parallax rate="0.3">
    <img src="header.png"></img>
  </parallax>

  <p>Site content continues</p>
</parallax-container>

Checkout a demo using the polyfill implementation.

Motivation

Having all of the contents of your page scroll at the same rate makes it appear very flat. Initially, fixed position backgrounds allowed having the content scroll independently past it. However, user interfaces have since started employing the use of parallax to create the appearance of 3d depth instead of either having content fixed or scrolling at full speed.

When this technique was brought to the web, it was implemented using script to position the parallaxing element in response to scroll updates. This worked reasonably well when scroll events were synchronous but every browser now implements asynchronous scrolling to ensure that scrolling remains smooth even when the page slows down. This means that often script positioned parallax will jitter when the browser is unable to service the script for every rendered frame.

Browser vendors have only recently started trying to define a specification which would make it possible to declaratively specify your parallax effect but there are many technical challenges that have made this a problem. However, it is possible to implement parallax in the browser's asynchronous scrolling today by combining 3d perspective and sticky positioning. This is very unergonomic, which is exactly the sort of case that permafills can solve!

Implementation

Parallax can be implemented in two ways, both of which make use of transforms:

  • Using perspective on the scrolling element to cause the shift in the scrolled parallax element to not be as great as the scrolled content.

  • Using sticky position with a transform (scale or perspective) to make the sticky position offset not actually shift the element as much as necessary to counteract the scroll.

The perspective implementation unfortunately doesn't work on mobile Safari with the webkit-overflow-scrolling: touch property, and sticky position is not yet implemented on the Edge browser. This means that using both approaches is necessary to achieve support on all major browsers.

API Surface

The simplest API surface supporting both of the implementations requires adding custom style to both the scrolling element and the element which is intended to parallax.

Variations and design considerations

  • In some circumstances, it can be difficult to calculate the correct parallax rate, i.e. as seen in the performant parallax demo where a background image is made to perfectly fill a gap slightly larger than that image. We could define a gap we want the parallax to fill between two elements.

  • In order be shifted by sticky positioning freely, the parallaxing elements need to be able to move by the scroll distance they must parallax over within their container, otherwise sticky position would constrain them. However, this container does not need to be the scroller. It may be nice to allow non top-level elements to parallax.