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.
if (XMLHttpRequest.prototype.overrideMimeType instanceof Function)
        return { output: 'Success', emulated: '' };
      else
        return { output: 'Failed', emulated: 'Yes' };
    },
    impact: 'Important',
    area: 'Viewer'
  },
  {
    id: 'XMLHttpRequest-response',
    name: 'XMLHttpRequest.prototype.response is present',
    run: function () {
      var xhr = new XMLHttpRequest();
      if ('response' in xhr)
        return { output: 'Success', emulated: '' };
      else
        return { output: 'Failed', emulated: 'Yes' };
    },
    impact: 'Critical',
    area: 'Core'
  },
  {
    id: 'bota',
    name: 'btoa() is present',
    run: function () {
      if ('btoa' in window)
        return { output: 'Success', emulated: '' };
      else
        return { output: 'Failed', emulated: 'Yes' };
    },