html {
  box-sizing: border-box;
}

*,
*::before,
*::after {
  box-sizing: inherit;
}

body {
  font-family: sans-serif;
  margin: 40px;
}

form {
  width: 370px;
  margin: 0 auto;
  padding: 40px;
  box-shadow: 0 4px 5px 0 rgba(0, 0, 0, 0.14),
    0 1px 10px 0 rgba(0, 0, 0, 0.12),
    0 2px 4px -1px rgba(0, 0, 0, 0.4);
}

label {
  display: block;
  margin-bottom: 24px;
  font-size: 18px;
}

input {
  border: 1px solid lightgray;
  padding: 8px;
  font-size: 18px;
  width: 100%;
  margin-top: 4px;
}

input:valid {
  border: 1px solid seagreen;
  background: honeydew;
}

button {
  display: block;
  padding: 20px 40px;
  margin: 0 auto;
  font-size: 18px;
  background: lightgrey;
  color: white;
  border: none;
}

form:valid button {
  background: steelblue;
}