Preload feature detection

preload_feature_detection.js

var DOMTokenListSupports = function(tokenList, token) {
  if (!tokenList || !tokenList.supports) {
    return;
  }
  try {
    return tokenList.supports(token);
  } catch (e) {
    if (e instanceof TypeError) {
      console.log("The DOMTokenList doesn't have a supported tokens list");
    } else {
      console.error("That shouldn't have happened");
    }
  }
};

var linkSupportsPreload = DOMTokenListSupports(document.createElement("link").relList, "preload");
if (!linkSupportsPreload) {
  // Dynamically load the things that relied on preload.
}

If you wanted to reduce the code bloat for production, would this do?

var supportsPreload = (function() { try { document.createElement("link").relList.supports('preload'); return 1 } catch(e) { return 0; }}());

@triblondon I think, better solution is: var supportsPreload = (function() { try { return document.createElement("link").relList.supports('preload'); } catch(e) { return false; }}());
https://github.com/filamentgroup/loadCSS/blob/master/src/cssrelpreload.js

Your code return false positive for FF 58.