.container {
  max-width: 400px;
  margin: auto;
  padding: 1em;
  border: 1px solid #ccc;
}

input:focus,
textarea:focus {
  border-color: blue;
  outline: none;
}

input:first-of-type {
  background-color: purple;
}

input:invalid,
select:invalid,
textarea:invalid {
  border: 2px solid red;
}

input:valid,
select:valid,
textarea:valid {
  border: 2px solid green;
}

button:hover {
  background-color: cyan;
}

.radio-group input[type="radio"]:checked {
border: 2px solid blue;
background-color: lightblue;
box-shadow: 0 0 5px blue;
}

.radio-group input[type="radio"]:checked + label {
  color: blue;
}