CSS Transitions in the Cascade Hierarchy

CSS transition declarations occupy a unique position in the CSS cascade hierarchy, sitting at the absolute top of the cascade’s origin and importance order while they are actively running. This article explains how the browser prioritizes active transitions over standard developer styles, the mechanics of property interpolation between cascade changes, and what happens to priority once an animation cycle completes.

The Cascade Order of Origin and Importance

To understand how transitions interact with developer (author) styles, it is necessary to examine the cascade levels defined by the CSS Cascading and Inheritance specification. When determining which style applies to an element, the browser sorts declarations in the following order of precedence, from lowest to highest:

  1. User agent normal declarations
  2. User normal declarations
  3. Author normal declarations (standard developer styles)
  4. CSS Animation declarations
  5. Author !important declarations
  6. User !important declarations
  7. User agent !important declarations
  8. CSS Transition declarations

Because active transition declarations reside at the eighth and final tier of this hierarchy, they override all other rules—including author !important styles—for the duration of the transition.

How Developer Styles Initiate Transitions

Transitions do not exist in isolation; they depend directly on developer styles to establish their start and end states.

A transition is triggered when an event (such as a class change, a pseudo-class like :hover, or an inline style modification) alters an element’s computed style. The browser evaluates the developer styles before the change and the developer styles after the change through the standard cascade rules (evaluating origin, specificity, and source order).

If a property difference exists between these two computed states and that property is listed under transition-property, the browser constructs a runtime transition declaration.

Behavior During Active Transitions

Once initiated, the transition generates animated, intermediate values that update on every frame. During this active phase:

  • Complete Precedence: The interpolating transition declaration takes precedence over the underlying author styles that initiated it. Even if an author rule uses !important or high-specificity selectors, the active transition value cannot be superseded by standard cascade updates to that specific property.
  • Property Independence: Only the properties actively being transitioned are elevated to the top of the cascade. Non-transitioned properties specified in the same rule set remain at their standard cascade levels (e.g., author normal or author !important).
  • Interrupted Transitions: If a developer style changes the target value while a transition is already in progress, the transition does not immediately snap to the new author style. Instead, the cascade recalculates the new destination from the current interpolated transition value, reversing or redirecting smoothly.

Post-Transition Resolution

As soon as the transition completes its duration (and any configured delay), the transition declaration is removed from the top of the cascade. At this point, the browser immediately yields control back to the standard cascade hierarchy. The element displays the computed value derived from author !important rules, author normal styles, or underlying CSS animations according to standard specificity rules.