<h1>Salutations, <span>my friend</span></h1>

h1 {
  display: block;
  font-size: 2em;
  color: hsl(207, 90%, 54%);
  text-decoration: underline solid blue 1px;
}

body {
  text-align: center;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
}

@supports not (text-decoration: solid blue 1px) {
  h1::after {    
    content: "Your browser doesn't support text-decoration-thckness in text-decoration.";
    display: block;
    color: #e43;
    margin-top: 15px;
    font-weight: bold;
    text-decoration: unset;
    font-size: 12px;
  }
}