Pseudo elements allow us to target elements that are not explicitly set in the DOM. Using ::before ::after we can actually create and manipulate elements in the DOM that do not impact the content. While ::first-letter ::first-line ::selection ::placeholder allow us to target elements that do not have a specific DOM element.

blockquote {
  quotes: "“" "”" "‘" "’";
}

blockquote::before {
  content: open-quote;
}
blockquote::after {
  content: close-quote;
}

About ::first-line, ::first-letter:

p::first-line {
  color: green;
}

p::first-letter {
  font-size
 
: 2em;
  float: left;
  line-height: 1.7em;
  padding: 0 .3em;
  font-weight: bold;
}

[CSS3] Target HTML Elements not Explicitly set in the DOM with CSS Pseudo Elements