html,
body {
    margin: 0;
    padding: 0;
    height: 100%;
}

header {
    background-color: #CD0A00;
    /* Coca-Cola red */
    display: flex;
    align-items: center;
    padding: 0.25rem 0.5rem;
    height: 40px;
}

button {
    background: none;
    border: none;
    color: white;
    font-size: 1.2rem;
    font-weight: bold;
    cursor: pointer;
}

iframe {
    width: 100%;
    height: calc(100% - 40px);
    /* restamos altura del header */
    border: none;
}