JSFiddle - React, Tailwind, and code Playground

HTML

<span id="spinner" class="pace-activity"></span>

CSS

.pace-activity {
    margin-top: 900px;
    display: block;
    z-index: 2000;
    width: 14px;
    height: 14px;
    border: solid 2px transparent;
    border-top-color: #29d;
    border-left-color: #29d;
    border-radius: 10px;
}
.add-anim {
    -webkit-animation: pace-spinner 400ms linear infinite;
    -moz-animation: pace-spinner 400ms linear infinite;
    -ms-animation: pace-spinner 400ms linear infinite;
    -o-animation: pace-spinner 400ms linear infinite;
    animation: pace-spinner 400ms linear infinite;
}
@-webkit-keyframes pace-spinner {
    0% {
        -webkit-transform: rotate(0deg);
        transform: rotate(0deg);
    }
    100% {
        -webkit-transform: rotate(360deg);
        transform: rotate(360deg);
    }
}
@-moz-keyframes pace-spinner {
    0% {
        -moz-transform: rotate(0deg);
        transform: rotate(0deg);
    }
    100% {
        -moz-transform: rotate(360deg);
        transform: rotate(360deg);
    }
}
@-o-keyframes pace-spinner {
    0% {
        -o-transform: rotate(0deg);
        transform: rotate(0deg);
    }
    100% {
        -o-transform: rotate(360deg);
        transform: rotate(360deg);
    }
}
@-ms-keyframes pace-spinner {
    0% {
        -ms-transform: rotate(0deg);
        transform: rotate(0deg);
    }
    100% {
        -ms-transform: rotate(360deg);
        transform: rotate(360deg);
    }
}
@keyframes pace-spinner {
    0% {
        transform: rotate(0deg);
        transform: rotate(0deg);
    }
    100% {
        transform: rotate(360deg);
        transform: rotate(360deg);
    }
}

JavaScript

$(window).on('scroll', function () {
    // console.log($(window).height());
    scroll_pos = $(window).scrollTop() + $(window).height();
    element_pos = $('#spinner').offset().top + $('#spinner').height();
    if (scroll_pos > element_pos) {
        $('#spinner').addClass('add-anim');
    };

});