style.css

* {
    font-family: Helvetica, sans-serif;
}

body {
    height: 100vh;
}

header {
    margin: 5em;
    text-align: center;
}

h1 {
    color: rgb(255, 123, 0);
}

h2 {
    font-size: 2em;
}

p {
    font-size: 1.5em;
    color: rgb(255, 123, 0);
}

main {
    height: 100vh;
    display: flex;
    flex-direction: column;
    margin-top: 10%;
    align-items: center;
    text-align: center;
}

input {
    font-size: 1.5em;
    text-align: center;
    color: rgb(255, 123, 0);
    padding: 0.5em;
    border: 2px solid black;
    border-radius: 50px;
    outline: none;
    caret-color: rgb(255, 123, 0);
}

button {
    width: 12em;
    font-size: 1.5em;
    background-color: rgb(255, 123, 0);
    color: white;
    padding: 0.5em;
    border-radius: 50px;
    outline: none;
    cursor: pointer;
}

.display--none {
    display: none;
}

.input--false {
    color: red;
    border-color: red;
    animation: error 0.1s 3 0s linear;
}

@keyframes error {
    0% {}
    25% {
        transform: translateX(5px);
    }
    50% {
        transform: translateX(0);
    }
    75% {
        transform: translateX(-5px);
    }
    100% {
        transform: rotate(xx) translateX(0);
    }
}