I have learned so many useless tricks about CSS, like this one.

You have to use different rules to style placeholder, otherwise, they will be ignored. Also reported by treehouse.

A stylus example to hide the placeholder when the learner class exists

  .learner
    &::-webkit-input-placeholder
      color white
    &:-moz-placeholder
      color white
    &::-moz-placeholder
      color white
    &:-ms-input-placeholder
      color white