main.css

body {
    margin: 0;
}

* {
    box-sizing: border-box;
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif
}

main {
    padding: 1em;
    width: 80%;
    margin: 0 auto;
    height: 100vh;
}

main h1 {
    margin-top: 0;
}

#controls {
    width: 75%;
    padding: 1em;
    border: 2px solid black;
    margin-bottom: 1em;
    float: left;
}

#inputs-container {
    display: flex;
    flex-wrap: wrap;
    justify-content: space-between;
    width: 100%;
}

#controls p {
    margin: 0 0 0.5em
}

#word-input-container {
    width: 75%;
    margin-right: 5%;
    height: 85%;
    float: right;
}

#word-input-container textarea {
    padding: 1em;
    height: 100%;
    width: -webkit-fill-available;
    width: 100%;
    border: 2px solid black;
    outline: none;
    resize: none;
}

#word-counter-container {
    width: 20%;
    padding: 1em;
    height: 95%;
    float: right;
    border: 2px solid black;
}

aside#word-counter-container h2 {
    margin-top: 0;
}

@media (max-width: 992px) {
    main {
        width: 100%;
    }
}

@media (max-width: 620px) {
    #controls {
        width: 100%;
    }
    
    #word-counter-container {
        width: 100%;
        display: block;
        height: auto;
    }

    #word-input-container {
        display: block;    
        width: 100%;
        margin: 1em 0 0;
        height: 50vh;
        float: left;
    }
}