Edit in JSFiddle

<div id="videoContainer" style="width: 100%;"></div>
<script>
document.addEventListener("DOMContentLoaded", function (t) {
    var e, o = "https://storage.googleapis.com/gpes-static/rutas-armas/RUTA_BURGOS_cuadrado.mp4", // Mobile version, small screens
        a = "https://storage.googleapis.com/gpes-static/rutas-armas/RUTA_BURGOS_720.mp4", // Desktop, large screens
        s = document.getElementById("videoContainer"),
        r = s.offsetWidth;
    e = r < 640 ? o : a;
    var d = document.createElement("video");
    d.setAttribute("controls", "controls"), d.style.maxWidth = "100%";
    var n = document.createElement("source");
    n.setAttribute("src", e), n.setAttribute("type", "video/mp4"), d.appendChild(n), s.appendChild(d)
});
</script>