Have you always used trial and error to style elements when it involves either a colon : or a pair of colon ::? Yeah, me too. But today is the day you become confident with your CSS pseudo-selection…whatever that means lol ;)
In this article, I explain them both with examples and share a few tips for styling elements using either.

The Difference
: precedes and identifies the state of an element while :: “creates” element(s). The difference between : and :: is that the former describes the state of a selected element usually involving user interaction while the latter is used to create element(s) as a part of the selected element and/or used to target elements using the selected element as reference.

It is important to note that : creates pseudo-classes, some examples are
:hover - to style an element when user is on it without selecting ie hovers over an element
:active - to style an element when clicked ie when element is active
:visited - to style anchor tags (links) when a user has clicked on it.
:focus - to style an input field that user clicked on.

While :: creates pseudo-elements. Some examples of pseudo-elements are
::before - targets created element that precedes selected element
::after - targets created element that succeeds selected element
::placeholder - targets placeholder atrribute value

How to Use
A good rule of thumb is to only use :: when it is necessary as this got introduced with CSS3 meaning :before is valid CSS and compatible with older browsers that haven’t fully adopted the changes introduced by CSS3. Following this rule makes your CSS backward-compatible.
It is also good practice to only use CSS-generated content when necessary as CSS-generated elements do not appear on the DOM and therefore cannot be parsed by accessibility tools.

I hope you found this helpful, thank for reading!