<div>Сначала <span class="botline1">подчеркивание</span> с auto.</div>
<div>Теперь <span class="botline2">вариант</span> для under.</div>
div { font-size: 55px; }
.botline1 {
  -webkit-text-decoration: red solid underline;
  text-decoration: red solid underline;
  -webkit-text-underline-position: auto;
  text-underline-position: auto;
}
.botline2 {
  -webkit-text-decoration: red solid underline;
  text-decoration: red solid underline;
  -webkit-text-underline-position: under;
  text-underline-position: under;
}
Rerun