HTMLify

*,
::before,
::after {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    display: grid;
    place-items: center;
    height: 100vh;
    background-color: #151515;
}

.share__btn {
    position: relative;
    display: flex;
    justify-content: center;
    align-items: center;
    width: 2em;
    height: 2em;
    border-radius: 100vmax;
    border: 0;
    font-size: 2rem;
    background-color: #fff;
    color: #151515;
    cursor: pointer;
    transition: width 300ms ease-in-out;
    transition-delay: 50ms;
    z-index: 1;
}

.share__btn::before {
    content: "x";
    position: absolute;
    left: 5px;
    font-size: 2.25rem;
    font-weight: 500;
    font-family: "Nunito", sans-serif;
    width: 1.5em;
    height: 1.5em;
    background-color: transparent;
    color: #fff;
    border-radius: inherit;
    z-index: -1;
    transition: background-color 300ms ease-in-out;
}

.social__icons {
    position: absolute;
    display: flex;
    justify-content: space-around;
    align-items: center;
    width: calc(100% - 2em);
    height: 100%;
    right: 0;
    border-radius: inherit;
}

.fa-facebook-f,
.fa-twitter,
.fa-instagram {
    transform: scale(0);
    transition: transform 300ms ease-in-out;
}

.share__icon {
    transform: scale(1);
    transition: transform 300ms ease-in-out;
}

.share__btn.open {
    width: 8em;
}

.share__btn.open::before {
    background-color: #F44336;
}

.share__btn.open :is(.fa-facebook-f, .fa-twitter, .fa-instagram) {
    transform: scale(1);
}

.share__btn.open .share__icon {
    transform: scale(0);
}