As MDN states:
In CSS 2, pseudo-elements were prefixed with a single colon character.
As pseudo-classes were also following the same convention, they were
indistinguishable. To solve this, CSS 2.1 changed the convention for
pseudo-elements. Now a pseudo-element is prefixed with two colon
characters, and a pseudo-class is still prefixed with a single colon.
As several browsers already implemented the CSS 2 version in a release
version, all browsers supporting the two-colon syntax also support the
old one-colon syntax.
If legacy browsers must be supported, :first-line is the only viable
choice; if not, ::first-line is preferred.
Further, as the W3 states:
This :: notation is introduced by the current document in order to
establish a discrimination between pseudo-classes and pseudo-elements.
For compatibility with existing style sheets, user agents must also
accept the previous one-colon notation for pseudo-elements introduced
in CSS levels 1 and 2 (namely, :first-line, :first-letter, :before and
:after). This compatibility is not allowed for the new pseudo-elements
introduced in this specification.