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h1 {
  display: inline-block;
  position: relative;
}

h1:after {
  content: "";
  position: absolute;
  height: 1px;
  background-color: red;
  width: 50%;
  left: 50%;
  transform: translate(-50%);
}

For fading effect at the edges, replace background-color with this:

  background: linear-gradient(
      to right, 
      transparent,
      red 40%,
      red 60%,
      transparent
  );