CSS :blank

The :blank pseudo-class has been proposed to match user-input elements whose value is empty. Browser support is limited, so it is not a safe production selector.

Syntax

input:blank {
  /* experimental */
}

Practical alternative

input:placeholder-shown {
  border-color: #94a3b8;
}

input:invalid {
  border-color: #ef4444;
}

Result:

Important note

For production forms, prefer :placeholder-shown, :valid, :invalid or JavaScript validation, depending on the behavior you need.