A simple JS function I use for lazy loading CSS files with Flask.

lazy.js

function lazyLoad(element, type, rel, link, crossOrigin = "anonymous", integrity = false) {
    var x = document.createElement(element);
    x.type = type;
    x.rel = rel;
    x.href = link;

    // subresource integrity for external content from different domains
    if (integrity) {
        x.crossOrigin = crossOrigin;
        x.integrity = integrity;
    }

    var s = document.getElementsByTagName(element)[0];
    s.parentNode.insertBefore(x, s);
}
//   lazyLoad('link', 'text/css', 'stylesheet', "{{ url_for('static', filename='main.css') }}");
//   lazyLoad('link', 'text/css', 'stylesheet', "https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css", crossorigin = "anonymous", integrity = "sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T");