A CSS pseudo-element is a keyword added to a selector that lets you style a specific part of the selected element(s).
Ref.: https://developer.mozilla.org/en-US/docs/Web/CSS/Pseudo-elements
custom : String -> List CSS.Internal.Property -> CSS.Internal.Class -> CSS.Internal.Class
before : List CSS.Internal.Property -> CSS.Internal.Class -> CSS.Internal.Class
In CSS, ::before creates a pseudo-element that is the first child of the selected element.
after : List CSS.Internal.Property -> CSS.Internal.Class -> CSS.Internal.Class
In CSS, ::after creates a pseudo-element that is the last child of the selected element.