On https://github.com/mozilla/pdf.js/wiki/Required-Browser-Features we mention required browser features for PDF.js, but I have the feeling that not all of them are necessary nowadays since we moved to using more modern features over the years. Moreover, as indicated in https://github.com/mozilla/pdf.js/issues/11950#issuecomment-637149085, some features may be missing such as range requests or `async/await`.

In short, we should verify which are still necessary and update the wiki page and the automated feature detection if possible.
var subfolders = fs.readdirSync(L10N_DIR);
  subfolders.sort();
  var viewerOutput = "";
  var locales = [];
  for (var i = 0; i < subfolders.length; i++) {
    var locale = subfolders[i];
    var dirPath = L10N_DIR + locale;
    if (!checkDir(dirPath)) {
      continue;
    }
    if (!/^[a-z][a-z]([a-z])?(-[A-Z][A-Z])?$/.test(locale)) {
      console.log("Skipping invalid locale: " + locale);
      continue;
    }
    mkdirp.sync(VIEWER_LOCALE_OUTPUT + "/" + locale);
    locales.push(locale);
    if (checkFile(dirPath + "/viewer.properties")) {
      viewerOutput +=
        "[" +
        locale +
        "]\n" +
        "@import url(" +
        locale +
        "/viewer.properties)\n\n";
    }
  }
  return merge([
    createStringSource("locale.properties", viewerOutput).pipe(
      gulp.dest(VIEWER_LOCALE_OUTPUT)
    ),