1.js

async function supportsWebp() {
  if (!self.createImageBitmap) return false;
  
  const webpData = 'data:image/webp;base64,UklGRhIAAABXRUJQVlA4TAYAAAAvQWxvAGs=';
  const blob = await fetch(webpData).then(r => r.blob());
  return createImageBitmap(blob).then(() => true, () => false);
}

addEventListener('install', event => {
  event.waitUntil(async function() {
    if (await supportsWebp()) {
      // …
    }
    else {
      // …
    }
  }());
});

It should do

Mmm iOS 14 beta 2 has no support for createImageBitmap, yet?

Actually not seeing any webp support of any kind yet, guess its still coming