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Why an element needs a starting style
A CSS transition interpolates between an element’s previous and new computed styles. When an element is rendered for the first time—or becomes visible after display: none—there may be no previous rendered state. The browser then has no before-value from which to run the transition. The @starting-style at-rule supplies that initial value for the entry transition.
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It is for transitions, not a prerequisite for CSS keyframe animations: a keyframe animation already defines its own starting keyframe. Use @starting-style when you want a transition between states and the entering element lacks a prior style state.
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Animate a newly appearing element
Set the final, visible values in the ordinary rule, declare the properties to transition, and put the entry values in @starting-style. This standalone form follows MDN’s documented syntax:
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.notice {
opacity: 1;
transform: translateY(0);
transition: opacity 180ms ease, transform 180ms ease;
}
@starting-style {
.notice {
opacity: 0;
transform: translateY(0.5rem);
}
}When .notice first appears, the starting values provide its before-state and the ordinary rule provides its destination. The browser transitions opacity and position between them. The duration and easing above are example author choices, not browser requirements.
Choose standalone or nested syntax
You can also nest @starting-style inside the selected rule. In this form, the starting declarations are scoped to that rule:
.panel {
opacity: 1;
transition: opacity 200ms;
@starting-style {
opacity: 0;
}
}Both forms are documented by MDN. With the standalone form, the starting-style selector has the same specificity as the corresponding ordinary selector. If declarations have equal specificity, put the standalone block after the normal rule so source order does not override the starting value.
Make display changes transition
display is a discrete property: it does not interpolate through intermediate values. To transition an element from display: none to a visible display value, include display in the transition and enable discrete transitions with allow-discrete. For example, a popover can use its open state as the destination:
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.popover {
opacity: 0;
transform: translateY(0.5rem);
display: none;
transition:
opacity 180ms ease,
transform 180ms ease,
display 180ms allow-discrete;
}
.popover:popover-open {
opacity: 1;
transform: translateY(0);
display: block;
}
@starting-style {
.popover:popover-open {
opacity: 0;
transform: translateY(0.5rem);
}
}The allow-discrete behavior lets display participate in the transition even though it does not interpolate. On entry, the visible display value takes effect at the start so the other properties can animate. The :popover-open selector is the destination state; the popover itself must be opened through the relevant HTML interaction or script.
Keep dialogs and popovers in the top layer during exit
Browsers manage whether modal dialogs and popovers belong to the top layer. For an exit transition, include overlay in the transition list with allow-discrete, as well as display. This delays removal from the top layer until the visual transition finishes:
.dialog {
opacity: 0;
transform: translateY(0.5rem);
display: none;
overlay: none;
transition:
opacity 180ms ease,
transform 180ms ease,
display 180ms allow-discrete,
overlay 180ms allow-discrete;
}
.dialog:open {
opacity: 1;
transform: translateY(0);
display: block;
overlay: auto;
}
@starting-style {
.dialog:open {
opacity: 0;
transform: translateY(0.5rem);
}
}Here :open represents the dialog’s open state. The dialog still needs to be opened and closed using the appropriate dialog behavior; CSS controls the transition rather than the interaction. The discrete display transition keeps the element rendered through its exit, while overlay delays its top-layer removal. See Chrome’s entry and exit animation examples for additional patterns and context.
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A starting style supplies the before-state for the initial appearance, or for a return from display: none. It does not become the element’s normal reverse state. When a shown element closes, it transitions from its visible values toward its ordinary hidden or default values. Entry and exit can therefore use different rules and need not be symmetrical.
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For a simple element whose visibility is controlled by a class, the same principle applies: the visible class is the destination, and the starting-style rule supplies the initial values when that state first renders. For more involved DOM changes, view transitions are another technique to consider; they solve a broader class of DOM-transition problems and are not automatically preferable to a state-based CSS transition.
Respect reduced-motion preferences
Some users ask their operating system or browser to reduce motion. Use prefers-reduced-motion: reduce to remove, shorten, or replace nonessential movement. A straightforward option is to disable the transition for those users:
@media (prefers-reduced-motion: reduce) {
.notice,
.popover,
.dialog {
transition: none;
}
}If a transition is useful for conveying a state change, you can instead replace a large movement with a less movement-intensive effect. MDN documents the preference and its use in CSS at the prefers-reduced-motion media feature reference.
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MDN classifies @starting-style as Baseline 2024 and says it has worked across the latest browser versions since August 2024. That is a broad compatibility milestone, not a minimum-version guarantee for every browser, embedded webview, or device. Check the compatibility data for the browsers your project supports, especially if older versions matter. See MDN’s @starting-style reference for the feature’s syntax, behavior, and compatibility information.
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