style.css

/* RESET */
* {
    border: 0;
    box-sizing: border-box;
    font: inherit;
    font-size: 100%;
    margin: 0;
    padding: 0;
    vertical-align: baseline;
}

body {
    line-height: 1;
}

a {
    text-decoration: none;
    color: inherit;
}

img {
    display: block;
    max-width: 100%;
}

p, h1, h2, h3, h4, h5, h6 {
    overflow-wrap: break-word;
}

ul {
    list-style: none;
}

/* STYLE */
body {
    background-color: hsl(204, 43%, 93%);
    color: hsl(0, 0%, 100%);
    display: flex;
    font-family: 'Karla', sans-serif;
    font-weight: 400;
    justify-content: center;
    min-height: 100vh;
}

main {
    border-radius: 10px;
    margin: 10vh auto;
    overflow: hidden;
    width: 315px;
}

article {
    background-color: hsl(0, 0%, 100%);
}

h1 {
    color: hsl(179, 62%, 43%);
    font-size: 1.4rem;
    font-weight: 700;
}

h2 {
    font-size: 1.2rem;
    font-weight: 700;
}

p, li {
    font-size: .9rem;
}

.second {
    color: hsl(71, 73%, 54%);
    font-size: 1.1rem;
    font-weight: 700;
    margin: 1.8rem auto;
}

#join {
    min-height: 33vh;
}

.third {
    color: hsl(180, 1%, 15%);
    line-height: 1.8;
}

#pricing {
    background-color: hsl(179, 62%, 43%);
    min-height: 28vh;
}

#pricing div {
    display: flex;
    flex-direction: row;
    margin: 1.2rem auto .7rem;
    align-items: center;
}

.pricing-dollar {
    font-size: 2.2em;
    margin-right: 1rem;
}

.pricing-month {
    font-size: 1rem;
}

.pricing-info {
    font-size: 1rem;
    margin: 1vh 0 3vh;
}

.signup {
    background-color: hsl(71, 73%, 54%);
    border: 0.1px;
    border-radius: 5px;
    box-shadow: 2px 2px 8px hsl(180, 1%, 15%);
    color: hsl(0, 0%, 100%);
    display: block;
    font-size: 1rem;
    padding: 2.5vh;
    text-align: center;
    width: 100%;
}

#features {
    min-height: 28vh;
    line-height: 1.5;
}

#features p {
    margin-top: 1.2rem;
}

/* Multiple assignments */
.cyan {
    background-color: hsl(179, 62%, 43%);
}

.pad {
    padding: 1.51rem;
}

.opacity {
    opacity: .8;
}

/* Media query */
@media screen and (min-width: 720px) {
    main {
        display: grid;
        grid-template-columns: repeat(2, 1fr);
        margin: auto;
        width: 640px;
    }
    .second {
        font-size: 1.1rem;
        margin: 1.8rem auto 1rem;
    }
    .third {
        font-size: 1rem;
    }
    #join {
        grid-column: 1 / span 2;
        min-height: 220px;
    }
    #pricing {
        grid-column: 1;
        min-height: 255px;
    }
    #features {
        grid-column: 2;
        min-height: 255px;
    }
    .pad {
        padding: 2.5rem;
    }
    #features p {
        margin-top: .9rem;
    }
}