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Understanding CSS Custom Properties: -sd-animation, -sd-duration, and -sd-easing

The snippet -sd-animation: sd-fadeIn; –sd-duration: 0ms; –sd-easing: ease-in; shows a pattern for using CSS custom properties (variables) to control animation behavior in a flexible, themeable way. This article explains what each part does, how they might be used, and provides practical examples for implementing them in real projects.

What these properties represent

  • -sd-animation: a custom property (likely vendor- or project-prefixed) that holds the name of an animation or an animation shorthand token (e.g., sd-fadeIn).
  • –sd-duration: a standard CSS custom property storing the animation duration (here 0ms).
  • –sd-easing: a custom property for the timing function (ease-in).

Using variables like these lets you change animation behavior at different scopes (global, component, or per-instance) without rewriting keyframes or animation declarations.

Example keyframes and usage

Define a reusable keyframe and a component style that reads the variables:

css
@keyframes sd-fadeIn {from { opacity: 0; transform: translateY(8px); }  to   { opacity: 1; transform: translateY(0); }}
/* component root /.component {  / defaults /  –sd-duration: 300ms;  –sd-easing: ease-out;  -sd-animation: sd-fadeIn;
  animation-name: var(-sd-animation);  animation-duration: var(–sd-duration);  animation-timing-function: var(–sd-easing);  animation-fill-mode: both;}

Override per-instance:

css
/ instant appearance /.card.instant {  –sd-duration: 0ms;  –sd-easing: ease-in;}
/ slower, bouncier intro */.card.slow {  –sd-duration: 600ms;  –sd-easing: cubic-bezier(.2,1,.3,1);}

Why use custom properties for animation?

  • Theming & scope: Easily alter animation timing across a theme or specific components.
  • Runtime adjustments: Update CSS variables from JavaScript for dynamic effects without toggling class names.
  • Maintainability: Keep keyframes and behavior centralized; only change variables where needed.

Notes and caveats

  • The property name -sd-animation is nonstandard but valid as a custom property if declared with the leading two hyphens (e.g., –sd-animation). Browsers ignore unknown properties in standard animation declarations unless used via var(). In the example above we used var(-sd-animation)—this should be var(–sd-animation) if following CSS custom property syntax.
  • Zero-duration (0ms) makes the animation effectively instantaneous; it still triggers any animation-fill-mode effects.
  • Ensure fallbacks if variables might be missing: animation-duration: var(–sd-duration, 300ms);

Quick reference

  • Use @keyframes to define the animation (e.g., sd-fadeIn).
  • Assign variables at appropriate scopes.
  • Apply animation properties using var(–name) with sensible defaults.

This approach yields flexible, composable animations that are easy to theme and adjust across a codebase.

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