text-decoration is a useful CSS shorthand

text-decoration is not just an on-or-off switch for underlines. It is shorthand for text-decoration-line, text-decoration-style, and text-decoration-color.

a {
  text-decoration: underline dotted blue;
}

This gives the link a blue, dotted underline in one declaration. You can write the three longhand properties separately when that is clearer, but the shorthand is handy for small rules like this one.