CSS prototype property is a Sciter specific extension.

When the engine computes styles it does [if needed] this (pseudocode):

   Object.setPrototypeOf(element, thatClass);
   element.componentDidMount();

There is no "performance sacrifice" in case of handling prototype properties.

Prototypes are switchable if needed:

  div.beer { prototype: Beer url(...); }
  div.beer:hover { prototype: BeerHovered url(...); }