On slot Elements

The element’s difference is similar to how variables in Sass are different from CSS custom properties: the former are static which means they are compiled away and the resulting CSS just contains the final value. The latter are dynamic: they remain active at runtime and changing the value of a custom property will affect any use of it. If you’ve used custom properties in CSS you might know that their being dynamic makes them extremely powerful. Slots likewise are dynamic.

Source: On <slot> Elements – Nathan Knowler