diff --git a/docs/999-big-list-of-options.md b/docs/999-big-list-of-options.md
index 202ffa95918..524f9245f41 100755
--- a/docs/999-big-list-of-options.md
+++ b/docs/999-big-list-of-options.md
@@ -486,6 +486,65 @@ Type: `boolean`
CLI: `--extend`/`--no-extend`
Default: `false`
Whether to extend the global variable defined by the `name` option in `umd` or `iife` formats. When `true`, the global variable will be defined as `(global.name = global.name || {})`. When false, the global defined by `name` will be overwritten like `(global.name = {})`.
+#### output.externalImportAssertions
+
+Type: `{[extName: string]: string | null} | false | (moduleInfo: ModulInfo) => ({[key: string]: string} | null)`
Default: `{".json": "json"}`
+
+Which import assertions to add to external imports in the output if the output format is `es`. By default, only external `.json` files will receive a `type: 'json'` assertion.
+
+- If an object is provided, then the keys are considered to be file extensions while the values add a corresponding `type` assertion to files of that type.
+
+ ```js
+ // config
+ export default {
+ external: ['./foo.css', './bar.json'],
+ // ...
+ output: {
+ externalImportAssertions: {
+ '.css': 'css'
+ }
+ // ...
+ }
+ };
+
+ // input
+ import './foo.css';
+ import './bar.json';
+
+ // output
+ import './foo.css' assert { type: 'css' };
+ import './bar.json';
+ ```
+
+- If a function is provided, it will be called once for each external dependency with information about the external module. If it returns an object, its key-value pairs will be added as arbitrary assertions to all imports of that dependency.
+
+ ```js
+ // config
+ export default {
+ external: ['./foo.css', './bar.json'],
+ // ...
+ output: {
+ externalImportAssertions({ id }) {
+ if (id.endsWith('.css')) {
+ return { type: 'css', special: 'attribute' };
+ }
+ return null;
+ }
+ // ...
+ }
+ };
+
+ // input
+ import './foo.css';
+ import './bar.json';
+
+ // output
+ import './foo.css' assert { type: 'css', special: 'attribute' };
+ import './bar.json';
+ ```
+
+- `false` will disable all assertions.
+
#### output.generatedCode
Type: `"es5" | "es2015" | { arrowFunctions?: boolean, constBindings?: boolean, objectShorthand?: boolean, preset?: "es5" | "es2015", reservedNamesAsProps?: boolean, symbols?: boolean }`
CLI: `--generatedCode `
Default: `"es5"`