<h1 class="underline">かちびと.net</h1>

* {
   box-sizing: border-box;
}
html,
body {
  margin: 0;
  padding: 0;
  height: 100%;
  width: 100%;
  background-color: #eee;
}
h1 {
  text-align: center;
  font-size: 48px;
  letter-spacing: 3px;
  color: #222;
  margin: 20px 0;
}
body {
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
}
.underline {
  position: relative;
}
.underline:hover,
.underline:focus,
.underline:active {
  cursor: pointer;
}
.underline:hover:after,
.underline:focus:after,
.underline:active:after {
  width: 100%;
}
.underline:after {
  content: '';
  position: absolute;
  left: 0;
  bottom: -3px;
  height: 3px;
  background-color: #000;
  width: 0;
  transition: width .4s;
}