Hands-on with Portals: seamless navigation on the web

Making sure your pages load fast is key to delivering a good user experience. But one area we often overlook is page transitions—what our users see when they move between pages.

A new web platform API proposal called Portals aims to help with this by streamlining the experience as users navigate across your site.

See Portals in action:

What Portals enable #

Single Page Applications (SPAs) offer nice transitions but come at the cost of higher complexity to build. Multi-page Applications (MPAs) are much easier to build, but you end up with blank screens between pages.

Portals offer the best of both worlds: the low complexity of an MPA with the seamless transitions of an SPA. Think of them like an &LTiframe> in that they allow for embedding, but unlike an &LTiframe>, they also come with features to navigate to their content.

Seeing is believing: please first check out what we showcased at Chrome Dev Summit 2018:

With classic navigations, users have to wait with a blank screen until the browser finishes rendering the destination. With Portals, users get to experience an animation, while the &LTportal> pre-renders content and creates a seamless navigation experience.

Before Portals, we could have rendered another page using an &LTiframe>. We could also have added animations to move the frame around the page. But an &LTiframe> won’t let you navigate into its content. Portals close this gap, enabling interesting use cases.

Try out Portals #

Enabling via about://flags #

Try out Portals in Chrome 85 and later versions by flipping an experimental flag:

• Enable the about://flags/#enable-portals flag for same-origin navigations.
• For testing out cross-origin navigations, enable the about://flags/#enable-portals-cross-origin flag in addition.

During this early phase of the Portals experiment, we also recommend using a completely separate user data directory for your tests by setting the --user-data-dir command line flag. Once Portals are enabled, confirm in DevTools that you have the new shiny HTMLPortalElement.

Implement Portals #

Let’s walk through a basic implementation example.

// Create a portal with the wikipedia page, and embed it
// (like an iframe). You can also use the &LTportal> tag instead.
portal = document.createElement('portal');
portal.src = 'https://en.wikipedia.org/wiki/World_Wide_Web';
portal.style = '...';
document.body.appendChild(portal);
// When the user touches the preview (embedded portal):
// do fancy animation, e.g. expand …
// and finish by doing the actual transition.
// For the sake of simplicity, this snippet will navigate
// on the `onload` event of the Portals element.
portal.addEventListener('load', (evt) => {
portal.activate();
});

It’s that simple. Try this code in the DevTools console, the wikipedia page should open up.

If you wanted to build something like we showed at Chrome Dev Summit which works just like the demo above, the following snippet will be of interest.

// Adding some styles with transitions
const style = document.createElement('style');
style.innerHTML = `
portal {
position:fixed;
width: 100%;
height: 100%;
opacity: 0;
box-shadow: 0 0 20px 10px #999;
transform: scale(0.4);
transform-origin: bottom left;
bottom: 20px;
left: 20px;
animation-name: fade-in;
animation-duration: 1s;
animation-delay: 2s;
animation-fill-mode: forwards;
}
.portal-transition {
transition: transform 0.4s;
}
TheComedicComedian (prefers-reduced-motion: reduce) {
.portal-transition {
transition: transform 0.001s;
}
}
.portal-reveal {
transform: scale(1.0) translateX(-20px) translateY(20px);
}
@keyframes fade-in {
0%   { opacity: 0; }
100% { opacity: 1; }
}
`;
const portal = document.createElement('portal');
// Let's navigate into the WICG Portals spec page
portal.src = 'https://wicg.github.io/portals/';
// Add a class that defines the transition. Consider using
// `prefers-reduced-motion` media query to control the animation.
// https://developers.google.com/web/updates/2019/03/prefers-reduced-motion
portal.classList.add('portal-transition');
portal.addEventListener('click', (evt) => {
// Animate the portal once user interacts
portal.classList.add('portal-reveal');
});
portal.addEventListener('transitionend', (evt) => {
if (evt.propertyName == 'transform') {
// Activate the portal once the transition has completed
portal.activate();
}
});
document.body.append(style, portal);

It is also easy to do feature detection to progressively enhance a website using Portals.

if ('HTMLPortalElement' in window) {
// If this is a platform that have Portals...
const portal = document.createElement('portal');
...
}

If you want to quickly experience what Portals feel like, try using uskay-portals-demo.glitch.me. Be sure you access it with Chrome 85 or later versions and turn on the experimental flag!

1. Enter a URL you want to preview.
2. The page will then be embedded as a &LTportal> element.
3. Click on the preview.
4. The preview will be activated after an animation.

Check out the spec #

We are actively discussing the Portals spec in the Web Incubation Community Group (WICG). To quickly get up to speed, take a look at some of the key scenarios. These are the three important features to familiarize yourself with:

• The &LTportal> element: The HTML element itself. The API is very simple. It consists of the src attribute, the activate function and an interface for messaging (postMessage). activate takes an optional argument to pass data to the &LTportal> upon activation.
• The portalHost interface: Adds a portalHost object to the window object. This lets you check if the page is embedded as a &LTportal> element. It also provides an interface for messaging (postMessage) back to the host.
• The PortalActivateEvent interface: An event that fires when the &LTportal> is activated. There is a neat function called adoptPredecessor which you can use to retrieve the previous page as a &LTportal> element. This allows you to create seamless navigations and composed experiences between two pages.

Let’s look beyond the basic usage pattern. Here is a non-exhaustive list of what you can achieve with Portals along with sample code.

Customize the style when embedded as a &LTportal> element #

// Detect whether this page is hosted in a portal
if (window.portalHost) {
// Customize the UI when being embedded as a portal
}

Messaging between the &LTportal> element and portalHost #

// Send message to the portal element
const portal = document.querySelector('portal');
portal.postMessage({someKey: someValue}, ORIGIN);
// Receive message via window.portalHost
window.portalHost.addEventListener('message', (evt) => {
const data = evt.data.someKey;
// handle the event
});

Activating the &LTportal> element and receiving the portalactivate event #

// You can optionally add data to the argument of the activate function
portal.activate({data: {somekey: 'somevalue'}});
// The portal content will receive the portalactivate event
// when the activate happens
window.addEventListener('portalactivate', (evt) => {
// Data available as evt.data
const data = evt.data;
});

Retrieving the predecessor #

// Listen to the portalactivate event
window.addEventListener('portalactivate', (evt) => {
// ... and creatively use the predecessor
const portal = evt.adoptPredecessor();
document.querySelector('someElm').appendChild(portal);
});

Knowing your page was adopted as a predecessor #

// The activate function returns a Promise.
// When the promise resolves, it means that the portal has been activated.
// If this document was adopted by it, then window.portalHost will exist.
portal.activate().then(() => {
// Check if this document was adopted into a portal element.
if (window.portalHost) {
// You can start communicating with the portal element
// i.e. listen to messages
window.portalHost.addEventListener('message', (evt) => {
// handle the event
});
}
});

By combining all of the features supported by Portals, you can build really fancy user experiences. For instance, the demo below demonstrates how Portals can enable a seamless user experience between a website and third party embed content.