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.
gulp
      .src(L10N_DIR + "/{" + locales.join(",") + "}/viewer.properties", {
        base: L10N_DIR,
      })
      .pipe(gulp.dest(VIEWER_LOCALE_OUTPUT)),
  ]);
});
gulp.task("cmaps", function (done) {
  var CMAP_INPUT = "external/cmaps";
  var VIEWER_CMAP_OUTPUT = "external/bcmaps";
  console.log();
  console.log("### Building cmaps");
  
  if (!checkFile(CMAP_INPUT + "/UniJIS-UCS2-H")) {
    console.log("./external/cmaps has no cmap files, download them from:");
    console.log("  https://github.com/adobe-type-tools/cmap-resources");
    throw new Error("cmap files were not found");
  }
  
  fs.readdirSync(VIEWER_CMAP_OUTPUT).forEach(function (file) {
    if (/\.bcmap$/i.test(file)) {
      fs.unlinkSync(VIEWER_CMAP_OUTPUT + "/" + file);
    }
  });
  var compressCmaps = require("./external/cmapscompress/compress.js")
    .compressCmaps;
  compressCmaps(CMAP_INPUT, VIEWER_CMAP_OUTPUT, true);
  done();
});
function preprocessCSS(source, mode, defines, cleanup) {