.compose-form__publish-button-wrapper button, .button[href = "/publish"] {
  text-indent: -9999px;
  line-height: 0; /* Collapse the original line */
}

.compose-form__publish-button-wrapper button:after, .button[href = "/publish"]:after {
  content:'Toot!'; 
  text-indent: 0;
  display: block;
  line-height: initial; /* New content takes up original line height */
}