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These look like custom CSS properties and a shorthand animation token used by a design system or UI library. Briefly:

• –sd-animation: sd-fadeIn;
  • A custom property holding the animation name or token (here “sd-fadeIn”), likely mapped by the library to an actual @keyframes animation or utility class that applies fade-in behavior.
• –sd-duration: 250ms;
  • Duration of the animation (250 milliseconds). Used by the library or consumed in a rule like animation-duration: var(–sd-duration).
• –sd-easing: ease-in;
  • Timing function for the animation. Used as animation-timing-function: var(–sd-easing).

How they’re typically used (example pattern):

• The design system defines keyframes and a class or style that composes them from variables:
  • @keyframes sd-fadeIn { from { opacity: 0; transform: translateY(4px);} to { opacity: 1; transform: translateY(0);} }
  • .sd-animate { animation-name: var(–sd-animation); animation-duration: var(–sd-duration); animation-timing-function: var(–sd-easing); animation-fill-mode: both; }

Notes:

• These are nonstandard CSS custom properties — they only do anything if the surrounding stylesheet or JS reads them.
• You can override them per element (inline or via selector) for flexible control.