Polyfills are required in order for a Svelte app to run in IE11. Here are the steps required:

1. Add the following in public/index.html before the script tag for bundle.js:

   <script defer src="https://polyfill.io/v3/polyfill.min.js"></script>

2. Add the following in rollup.config.js just before the line production && terser():

   babel({
     extensions: ['.js', '.mjs', '.html', '.svelte'],
     exclude: ['node_modules/@babel/**', 'node_modules/core-js/**'],
     presets: [
       [
         '@babel/preset-env',
         {
           targets: {
             ie: '11'
           },
           useBuiltIns: 'usage',
           corejs: 3
         }
       ]
     ],
     plugins: [
       '@babel/plugin-syntax-dynamic-import',
       [
         '@babel/plugin-transform-runtime',
         {
           useESModules: true
         }
       ]
     ],
     runtimeHelpers: true
   }),

3. Enter npm install -D for each of these packages:

   • @babel/plugin-syntax-dynamic-import
   • @babel/plugin-transform-runtime
   • @babel/runtime
   • @rollup/plugin-babel
   • corejs
   • whatwg-fetch

4. In each source file where the Fetch API is being used, add the following:

   import 'whatwg-fetch';

5. Remove all use of CSS variables.

6. Test all uses of CSS flexbox and grid layout. There are some bugs in the IE11 implementation of these.

Unfortunately all the polyfills make the generated bundle.js file much bigger. For one app the size changed from 52K to 410K.