content-visibility property, launching in Chromium 85, might be one of the most impactful new CSS properties for improving page load performance.
content-visibility enables the user agent to skip an element's rendering work, including layout and painting, until it is needed. Because rendering is skipped, if a large portion of your content is off-screen, leveraging the
content-visibility property makes the initial user load much faster. It also allows for faster interactions with the on-screen content. Pretty neat.
Browser support #
content-visibility relies on primitives within the the CSS Containment Spec. While
content-visibility is only supported in Chromium 85 for now (and deemed "worth prototyping" for Firefox), the Containment Spec is supported in most modern browsers.
CSS Containment #
The key and overarching goal of CSS containment is to enable rendering performance improvements of web content by providing predictable isolation of a DOM subtree from the rest of the page.
Basically a developer can tell a browser what parts of the page are encapsulated as a set of content, allowing the browsers to reason about the content without needing to consider state outside of the subtree. Knowing which bits of content (subtrees) contain isolated content means the browser can make optimization decisions for page rendering.
There are four types of CSS containment, each a potential value for the
contain CSS property, which can be combined together in a space-separated list of values:
size: Size containment on an element ensures that the element's box can be laid out without needing to examine its descendants. This means we can potentially skip layout of the descendants if all we need is the size of the element.
layout: Layout containment means that the descendants do not affect the external layout of other boxes on the page. This allows us to potentially skip layout of the descendants if all we want to do is lay out other boxes.
style: Style containment ensures that properties which can have effects on more than just its descendants don't escape the element (e.g. counters). This allows us to potentially skip style computation for the descendants if all we want is to compute styles on other elements.
paint: Paint containment ensures that the descendants of the containing box don't display outside its bounds. Nothing can visibly overflow the element, and if an element is off-screen or otherwise not visible, its descendants will also not be visible. This allows us to potentially skip painting the descendants if the element is offscreen.
Skipping rendering work with
It may be hard to figure out which containment values to use, since browser optimizations may only kick in when an appropriate set is specified. You can play around with the values to see what works best, or you can use another CSS property called
content-visibility to apply the needed containment automatically.
content-visibility ensures that you get the largest performance gains the browser can provide with minimal effort from you as a developer.
The content-visibility property accepts several values, but
auto is the one that provides immediate performance improvements. An element that has
content-visibility: auto gains
paint containment. If the element is off-screen (and not otherwise relevant to the user—relevant elements would be the ones that have focus or selection in their subtree), it also gains
size containment (and it stops painting and hit-testing its contents).
What does this mean? In short, if the element is off-screen its descendants are not rendered. The browser determines the size of the element without considering any of its contents, and it stops there. Most of the rendering, such as styling and layout of the element's subtree are skipped.
As the element approaches the viewport, the browser no longer adds the
size containment and starts painting and hit-testing the element's content. This enables the rendering work to be done just in time to be seen by the user.
A note on accessibility #
One of the features of
content-visibility: auto is that the off-screen content remains available in the document object model and therefore, the accessibility tree (unlike with
visibility: hidden). This means, that content can be searched for on the page, and navigated to, without waiting for it to load or sacrificing rendering performance.
The flip-side of this, however, is that landmark elements with style features such as
display: none or
visibility: hidden will also appear in the accessibility tree when off-screen, since the browser will not render these styles until they enter the viewport. To prevent these from being visible in the accessibility tree, potentially causing clutter, be sure to also add
Caution: In Chromium 85-89, off-screen children within
content-visibility: auto were marked as invisible. In particular, headings and landmark roles were not exposed to accessibility tools. In Chromium 90 this was updated so that they are exposed.
Example: a travel blog #
A travel blog typically contains a set of stories with a few pictures, and some descriptive text. Here is what happens in a typical browser when it navigates to a travel blog:
- A part of the page is downloaded from the network, along with any needed resources.
- The browser styles and lays out all of the contents of the page, without considering if the content is visible to the user.
- The browser goes back to step 1 until all of the page and resources are downloaded.
In step 2, the browser processes all of the contents looking for things that may have changed. It updates the style and layout of any new elements, along with the elements that may have shifted as a result of new updates. This is rendering work. This takes time.
Now consider what happens if you put
content-visibility: auto on each of the individual stories in the blog. The general loop is the same: the browser downloads and renders chunks of the page. However, the difference is in the amount of work that it does in step 2.
With content-visibility, it will style and layout all of the contents that are currently visible to the user (they are on-screen). However, when processing the story that is fully off-screen, the browser will skip the rendering work and only style and layout the element box itself.
The performance of loading this page would be as if it contained full on-screen stories and empty boxes for each of the off-screen stories. This performs much better, with expected reduction of 50% or more from the rendering cost of loading. In our example, we see a boost from a 232ms rendering time to a 30ms rendering time. That's a 7x performance boost.
What is the work that you need to do in order to reap these benefits? First, we chunk the content into sections:
Then, we apply the following style rule to the sections:
contain-intrinsic-size: 1000px; /* Explained in the next section. */
Note that as content moves in and out of visibility, it will start and stop being rendered as needed. However, this does not mean that the browser will have to render and re-render the same content over and over again, since the rendering work is saved when possible.
Specifying the natural size of an element with
In order to realize the potential benefits of
content-visibility, the browser needs to apply size containment to ensure that the rendering results of contents do not affect the size of the element in any way. This means that the element will lay out as if it was empty. If the element does not have a height specified in a regular block layout, then it will be of 0 height.
This might not be ideal, since the size of the scrollbar will shift, being reliant on each story having a non-zero height.
Thankfully, CSS provides another property,
contain-intrinsic-size, which effectively specifies the natural size of the element if the element is affected by size containment. In our example, we are setting it to
1000px as an estimate for the height and width of the sections.
This means it will lay out as if it had a single child of "intrinsic-size" dimensions, ensuring that your unsized divs still occupy space.
contain-intrinsic-size acts as a placeholder size in lieu of rendered content.
Hiding content with
content-visibility: hidden #
What if you want to keep the content unrendered regardless of whether or not it is on-screen, while leveraging the benefits of cached rendering state? Enter:
content-visibility: hidden property gives you all of the same benefits of unrendered content and cached rendering state as
content-visibility: auto does off-screen. However, unlike with
auto, it does not automatically start to render on-screen.
This gives you more control, allowing you to hide an element's contents and later unhide them quickly.
Compare it to other common ways of hiding element's contents:
display: none: hides the element and destroys its rendering state. This means unhiding the element is as expensive as rendering a new element with the same contents.
visibility: hidden: hides the element and keeps its rendering state. This doesn't truly remove the element from the document, as it (and it's subtree) still takes up geometric space on the page and can still be clicked on. It also updates the rendering state any time it is needed even when hidden.
content-visibility: hidden, on the other hand, hides the element while preserving its rendering state, so, if there are any changes that need to happen, they only happen when the element is shown again (i.e. the
content-visibility: hidden property is removed).
Some great use cases for
content-visibility: hidden are when implementing advanced virtual scrollers, and measuring layout. They're also great for single-page applications (SPA's). Inactive app views can be left in the DOM with
content-visibility: hidden applied to prevent their display but maintain their cached state. This makes the view quick to render when it becomes active again.
In an experiment, Facebook engineers observed an up to 250ms improvement in navigation times when going back to previously cached views.
content-visibility and the CSS Containment Spec mean some exciting performance boosts are coming right to your CSS file. For more information on these properties, check out: