Before we begin, let’s take a quick look at the following video and imagine how much CSS and JavaScript would take to create an animation like this. Notice that the cart counter is also animated, and the animation runs right after the previous one completes.

Although this animation looks alright, it’s just a minor improvement. Currently, the API doesn’t really know that the image (shared element) that is being moved from the container to the overlay is the same element in their respective states. We need to instruct the browser to pay special attention to the image element when switching between states, so let’s do that!

With page-transition-tag CSS property, we can easily tell the browser to watch for the element in both outgoing and incoming images, keep track of element’s size and position that are changing between them, and apply the appropriate animation.

Another important caveat is that page-transition-tag has to be unique, and we can apply it to only one element during the duration of the animation. This is why we apply it to the active image element right before the image is moved to the overlay and remove it when the image overlay is closed and the image is returned to its original position:

Alternatively, we could have used JavaScript to toggle the page-transition-tag property inline on the element. However, it’s better to use the CSS class toggle to make use of media queries to apply the tag conditionally:

And that’s pretty much it! Let’s take a look at our example with the shared element applied:

We’ve created this complex transition with just a few lines of CSS and JavaScript, which turned out great. However, we expect to have more control over the animation properties like duration, easing function, delay, and so on to create even more elaborate animations or compose them for even greater effect.

Shared Element Transitions API makes use of CSS animation properties and we can use them to fully customize our state animation. But which CSS selectors to use for these outgoing and incoming states that the API is generating for us?

For example, when we apply the page-transition-tag: active-image, its pseudo-elements are added to the tree:

In our example, we want to modify both the crossfade (root) animation and the shared element animation. We can use the universal selector * with the pseudo-element to change animation properties for all available transition elements and target pseudo-elements for specific animation using the page-transition-tag value.

In this example, we are applying 400ms duration for all animated elements with an ease-in-out easing function, and then override the active-image transition easing function and setting a custom cubic-bezier value:

The following code snippet turns off animations for all elements using the Shared Element Transitions API. This is a shotgun solution, and we need to ensure that DOM updates smoothly and remains usable even with the animations turned off:

Now, all we have to do is assign the exit animation to the outgoing image pseudo-element and the entry animation to the incoming image pseudo-element. We can set a page-transition-tag directly to the HTML image element as it’s the only element that will perform this animation:

Even the seemingly simple crossfade animations can look cool, don’t you think? I think this particular animation fits really nicely with the dark theme we have in the example.

And that is it! It amazes me every time how elaborate these animations can turn out with so few lines of additional code, all thanks to Shared Element Transitions API. Notice that the header element with the cart icon is fixed, so it sticks to the top, and our standard animation setup works like a charm, regardless!

In the next article, we’ll explore the API’s potential when it comes to transition between different pages in Single Page Apps (SPA) and the upcoming Cross-document same-origin transitions, which are yet to be implemented.

Go ahead and build something awesome!