Prior to Modernizr v3, I was using yepnope.js
Modernizr.load and yepnope.js have both been deprecated. How do we conditionally call stylesheets or javascript files n
Have a look here for scripts: https://stackoverflow.com/a/7719185 — no jQuery required. (Note that there's a shorter Promise example a bit down in the answer; don't stop reading after the first example (like I did)).
And here for CSS: loadCSS: https://github.com/filamentgroup/loadCSS/blob/master/src/loadCSS.js
— this is, in a way, even better than YepNope.js, if you don't need any of its features except for just loading stuff + callback. Because the stuff linked above, is smaller (smaller min.js.gz) than yepnope.js.
(And can combine with Modernizr like mhk showed in his answer.)