Limited availability

This feature is not Baseline because it does not work in some of the most widely-used browsers.

Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The overlay CSS property specifies whether an element appearing in the top layer (for example, a shown popover or modal <dialog> element) is actually rendered in the top layer. This property is only relevant within a list of transition-property values, and only if allow-discrete is set as the transition-behavior.

It is important to note that overlay can only be set by the browser — author styles cannot change the overlay value of any element. You can, however, add overlay to the list of transition properties set on an element. This causes its removal from the top layer to be deferred so it can be animated instead of disappearing immediately.

Note: When transitioning overlay, you need to set transition-behavior: allow-discrete on the transition so that it will animate. overlay animations differ from normal discrete animations in that the visible (i.e. auto) state will always be shown for the full duration of the transition, regardless of whether it is the start or end state.


/* Keyword values */
overlay: auto;
overlay: none;

/* Global values */
display: inherit;
display: initial;
display: revert;
display: revert-layer;
display: unset;



The element is rendered in the top layer if it is promoted to the top layer.


The element is not rendered in the top layer.

Formal definition

Initial valuenone
Applies toall elements
Computed valueas specified
Animation typeDiscrete behavior except when animating to or from none is visible for the entire duration

Formal syntax

overlay = 
none |


Transitioning a popover

In this example, a popover is animated as it transitions from hidden to shown and back again.


The HTML contains a <div> element declared as a popover using the popover attribute, and a <button> element designated as the popover's display control using its popovertarget attribute.

<button popovertarget="mypopover">Show the popover</button>
<div popover="auto" id="mypopover">I'm a Popover! I should animate.</div>


The overlay property is only present in the list of transitioned properties. As overlay is a user-agent controlled property, it is not declared in the pre-transition or post-transition states.

html {
  font-family: Arial, Helvetica, sans-serif;

[popover]:popover-open {
  opacity: 1;
  transform: scaleX(1);

[popover] {
  font-size: 1.2rem;
  padding: 10px;

  /* Final state of the exit animation */
  opacity: 0;
  transform: scaleX(0);

    opacity 0.7s,
    transform 0.7s,
    overlay 0.7s allow-discrete,
    display 0.7s allow-discrete;
  /* Equivalent to
  transition: all 0.7s allow-discrete; */

/* Needs to be included after the previous [popover]:popover-open
   rule to take effect, as the specificity is the same */
@starting-style {
  [popover]:popover-open {
    opacity: 0;
    transform: scaleX(0);

/* Transition for the popover's backdrop */

[popover]::backdrop {
  background-color: rgb(0 0 0 / 0%);
    display 0.7s allow-discrete,
    overlay 0.7s allow-discrete,
    background-color 0.7s;
  /* Equivalent to
  transition: all 0.7s allow-discrete; */

[popover]:popover-open::backdrop {
  background-color: rgb(0 0 0 / 25%);

/* Nesting selectors (&) cannot represent pseudo-elements, so this 
   starting-style rule cannot be nested. */

@starting-style {
  [popover]:popover-open::backdrop {
    background-color: rgb(0 0 0 / 0%);

The two properties we want to animate are opacity and transform): we want the popover to fade in and out while growing and shrinking in the horizontal direction. We set a starting state for these properties on the default hidden state of the popover element (selected via [popover]), and an end state on the open state of the popover (selected via the :popover-open pseudo-class). We then set a transition property to animate between the two.

Because the animated element is being promoted to the top layer when shown and removed from the top layer when hidden, overlay is added to the list of transitioned elements. This ensures that the removal of the element from the top layer is deferred until the animation has ended. This doesn't make a huge difference for simple animations such as this one, but in more complex cases not doing this can result in the element being removed from the overlay too quickly, meaning the animation is not smooth or effective. Note that the transition-behavior: allow-discrete value is also set in the shorthand to enable discrete transitions.

The following steps are also required to get the animation working in both directions:

  • A starting state for the animation is set inside the @starting-style at-rule. This is needed to avoid unexpected behavior. By default transitions are not triggered on elements' first style updates, or when the display type changes from none to another type. @starting-style allows you to override that default in a specific controlled fashion. Without this, the entry animation would not occur and the popover would just appear.
  • display is also added to the list of transitioned elements so that the animated element is visible (set to display: block) throughout both the entry and exit animation. Without this, the exit animation would not be visible; in effect, the popover would just disappear. Again, transition-behavior: allow-discrete is required in this case for the animation to occur.

You'll note that we've also included a transition on the ::backdrop that appears behind the popover when it opens, to provide a nice darkening animation. [popover]:popover-open::backdrop is needed to select the backdrop when the popover is open.


The code renders as follows:

Note: Because popovers change from display: none to display: block each time they are shown, the popover transitions from its @starting-style styles to its [popover]:popover-open styles every time the entry transition occurs. When the popover closes, it transitions from its [popover]:popover-open state to the default [popover] state.

It is possible for the style transition on entry and exit to be different in such cases. See our Demonstration of when starting styles are used example for a proof of this.


CSS Positioned Layout Module Level 4
# overlay

Browser compatibility

BCD tables only load in the browser

See also