The ::before and ::after pseudo-elements in CSS allows you to insert content onto a page without it needing to be in the HTML.
Every browser that supports the double colon (::) CSS3 syntax also supports just the (:) syntax, but Internet Explorer (IE) 8 only supports the single-colon, so for now, it’s recommended to just use the single-colon for best browser support.
:: is the newer format intended to distinguish pseudo content from pseudo-selectors. If you don’t need IE 8 support, feel free to use the double-colon.