.fieldset {
  position: relative;
  border: 1px solid #ddd;
  padding: 10px;
}

.fieldset h1 {
  position: absolute;
  top: 0;
  font-size: 18px;
  line-height: 1;
  margin: -9px 0 0; /* half of font-size */
  background: #fff;
  padding: 0 3px;
}