Can I apply a style to all pseudo selectors in CSS or Sass?

Short Answer: No, not directly

However, a mixin could be used to have a similar effect.

// Sets the style only for pseudo selectors
@mixin setLinkSelectorStyle {
  &:hover, &:link, &:active, &:visited {
        @content;
    }
}

// Sets the style to pseudo selectors AND base default anchor
@mixin setLinkStyleAll {
  &, &:hover, &:link, &:active, &:visited {
        @content;
    }
}

a {
  color:red;
  @include setLinkSelectorStyle {
    color:gold;
  }
}

a.specialLink {
  @include setLinkStyleAll {
    color:purple;
  }
}

[Example using http://sassmeister.com/ compiled SASS]

a {
  color: red;
}
a:hover, a:link, a:active, a:visited {
  color: gold;
}

a.specialLink, a.specialLink:hover, a.specialLink:link, a.specialLink:active, a.specialLink:visited {
  color: purple;
}

<a>Normal anchor, No href (:link won't work, but other selectors will)</a>
<hr />
<a href="#">Normal anchor</a>
<hr />
<a class="specialLink">Specific class (no href)</a>
<hr />
<a class="specialLink" href="#">Specific class</a>

The mixins will create a rule for all the pseudo selectors when the mixin is included on the anchor / class.

Removed old answer, look at history to see it.

U can do it like that:

  &:before, &:after {
      content: 'a';
  }