https://mvolkmann.github.io/blog/svelte/supporting-ie11/

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

babel({
  babelHelpers: 'runtime',
  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
      }
    ]
  ]
}),

npm install --save @babel/core @babel/runtime
npm install --save babel
npm install --save @babel/node
npm install --save @babel/plugin-syntax-dynamic-import
npm install --save @babel/plugin-transform-runtime
npm install --save @babel/preset-env
npm install --save @babel/plugin-syntax-dynamic-import @babel/plugin-transform-runtime @babel/runtime @rollup/plugin-babel corejs whatwg-fetch 
npm install --save sveltestrap
npm install --save svelte-spa-router
npm install --save rollup-plugin-postcss

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

  import 'whatwg-fetch';