how to check if any form field is empty of not in html css code example

Example: check if input is empty css

/* If input is not empty */
input:not(:placeholder-shown) {
  /* You need to add a placeholder to your fields. For example: <input "placeholder=" "/> */
  border-color: green;
}

/* If input is empty */
input:placeholder-shown {
  border-color: red;
}