diff --git a/lib/productions/callback-interface.js b/lib/productions/callback-interface.js
index da9dc96f..a22b0098 100644
--- a/lib/productions/callback-interface.js
+++ b/lib/productions/callback-interface.js
@@ -5,8 +5,11 @@ import { Constant } from "./constant.js";
 export class CallbackInterface extends Container {
   /**
    * @param {import("../tokeniser.js").Tokeniser} tokeniser
+   * @param {*} callback
+   * @param {object} [options]
+   * @param {import("./container.js").AllowedMember[]} [options.extMembers]
    */
-  static parse(tokeniser, callback, { partial = null } = {}) {
+  static parse(tokeniser, callback, { extMembers = [] } = {}) {
     const tokens = { callback };
     tokens.base = tokeniser.consume("interface");
     if (!tokens.base) {
@@ -16,8 +19,8 @@ export class CallbackInterface extends Container {
       tokeniser,
       new CallbackInterface({ source: tokeniser.source, tokens }),
       {
-        inheritable: !partial,
         allowedMembers: [
+          ...extMembers,
           [Constant.parse],
           [Operation.parse, { regular: true }],
         ],
diff --git a/lib/productions/container.js b/lib/productions/container.js
index d52dc509..8f1f22d5 100644
--- a/lib/productions/container.js
+++ b/lib/productions/container.js
@@ -16,6 +16,19 @@ function inheritance(tokeniser) {
   return { colon, inheritance };
 }
 
+/**
+ * Parser callback.
+ * @callback ParserCallback
+ * @param {import("../tokeniser.js").Tokeniser} tokeniser
+ * @param {...*} args
+ */
+
+/**
+ * A parser callback and optional option object.
+ * @typedef AllowedMember
+ * @type {[ParserCallback, object?]}
+ */
+
 export class Container extends Base {
   /**
    * @param {import("../tokeniser.js").Tokeniser} tokeniser
diff --git a/lib/productions/dictionary.js b/lib/productions/dictionary.js
index fa55d979..57462c4c 100644
--- a/lib/productions/dictionary.js
+++ b/lib/productions/dictionary.js
@@ -5,9 +5,10 @@ export class Dictionary extends Container {
   /**
    * @param {import("../tokeniser.js").Tokeniser} tokeniser
    * @param {object} [options]
+   * @param {import("./container.js").AllowedMember[]} [options.extMembers]
    * @param {import("../tokeniser.js").Token} [options.partial]
    */
-  static parse(tokeniser, { partial } = {}) {
+  static parse(tokeniser, { extMembers = [], partial } = {}) {
     const tokens = { partial };
     tokens.base = tokeniser.consume("dictionary");
     if (!tokens.base) {
@@ -18,7 +19,7 @@ export class Dictionary extends Container {
       new Dictionary({ source: tokeniser.source, tokens }),
       {
         inheritable: !partial,
-        allowedMembers: [[Field.parse]],
+        allowedMembers: [...extMembers, [Field.parse]],
       }
     );
   }
diff --git a/lib/productions/interface.js b/lib/productions/interface.js
index e22b54a2..0f88e77c 100644
--- a/lib/productions/interface.js
+++ b/lib/productions/interface.js
@@ -34,8 +34,12 @@ function static_member(tokeniser) {
 export class Interface extends Container {
   /**
    * @param {import("../tokeniser.js").Tokeniser} tokeniser
+   * @param {import("../tokeniser.js").Token} base
+   * @param {object} [options]
+   * @param {import("./container.js").AllowedMember[]} [options.extMembers]
+   * @param {import("../tokeniser.js").Token|null} [options.partial]
    */
-  static parse(tokeniser, base, { partial = null } = {}) {
+  static parse(tokeniser, base, { extMembers = [], partial = null } = {}) {
     const tokens = { partial, base };
     return Container.parse(
       tokeniser,
@@ -43,6 +47,7 @@ export class Interface extends Container {
       {
         inheritable: !partial,
         allowedMembers: [
+          ...extMembers,
           [Constant.parse],
           [Constructor.parse],
           [static_member],
diff --git a/lib/productions/mixin.js b/lib/productions/mixin.js
index c3094e7c..19cce9f8 100644
--- a/lib/productions/mixin.js
+++ b/lib/productions/mixin.js
@@ -6,14 +6,13 @@ import { stringifier } from "./helpers.js";
 
 export class Mixin extends Container {
   /**
-   * @typedef {import("../tokeniser.js").Token} Token
-   *
    * @param {import("../tokeniser.js").Tokeniser} tokeniser
-   * @param {Token} base
+   * @param {import("../tokeniser.js").Token} base
    * @param {object} [options]
-   * @param {Token} [options.partial]
+   * @param {import("./container.js").AllowedMember[]} [options.extMembers]
+   * @param {import("../tokeniser.js").Token} [options.partial]
    */
-  static parse(tokeniser, base, { partial } = {}) {
+  static parse(tokeniser, base, { extMembers = [], partial } = {}) {
     const tokens = { partial, base };
     tokens.mixin = tokeniser.consume("mixin");
     if (!tokens.mixin) {
@@ -24,6 +23,7 @@ export class Mixin extends Container {
       new Mixin({ source: tokeniser.source, tokens }),
       {
         allowedMembers: [
+          ...extMembers,
           [Constant.parse],
           [stringifier],
           [Attribute.parse, { noInherit: true }],
diff --git a/lib/productions/namespace.js b/lib/productions/namespace.js
index ef7c35f5..34fa6e49 100644
--- a/lib/productions/namespace.js
+++ b/lib/productions/namespace.js
@@ -9,9 +9,10 @@ export class Namespace extends Container {
   /**
    * @param {import("../tokeniser.js").Tokeniser} tokeniser
    * @param {object} [options]
+   * @param {import("./container.js").AllowedMember[]} [options.extMembers]
    * @param {import("../tokeniser.js").Token} [options.partial]
    */
-  static parse(tokeniser, { partial } = {}) {
+  static parse(tokeniser, { extMembers = [], partial } = {}) {
     const tokens = { partial };
     tokens.base = tokeniser.consume("namespace");
     if (!tokens.base) {
@@ -22,6 +23,7 @@ export class Namespace extends Container {
       new Namespace({ source: tokeniser.source, tokens }),
       {
         allowedMembers: [
+          ...extMembers,
           [Attribute.parse, { noInherit: true, readonly: true }],
           [Constant.parse],
           [Operation.parse, { regular: true }],
diff --git a/lib/productions/operation.js b/lib/productions/operation.js
index b92c10aa..380ada5e 100644
--- a/lib/productions/operation.js
+++ b/lib/productions/operation.js
@@ -9,12 +9,10 @@ import { validationError } from "../error.js";
 
 export class Operation extends Base {
   /**
-   * @typedef {import("../tokeniser.js").Token} Token
-   *
    * @param {import("../tokeniser.js").Tokeniser} tokeniser
    * @param {object} [options]
-   * @param {Token} [options.special]
-   * @param {Token} [options.regular]
+   * @param {import("../tokeniser.js").Token} [options.special]
+   * @param {import("../tokeniser.js").Token} [options.regular]
    */
   static parse(tokeniser, { special, regular } = {}) {
     const tokens = { special };
diff --git a/lib/webidl2.js b/lib/webidl2.js
index 6129879b..26a020ad 100644
--- a/lib/webidl2.js
+++ b/lib/webidl2.js
@@ -12,11 +12,22 @@ import { CallbackInterface } from "./productions/callback-interface.js";
 import { autoParenter } from "./productions/helpers.js";
 import { Eof } from "./productions/token.js";
 
+/** @typedef {'callbackInterface'|'dictionary'|'interface'|'mixin'|'namespace'} ExtendableInterfaces */
+/** @typedef {{ extMembers?: import("./productions/container.js").AllowedMember[]}} Extension */
+/** @typedef {Partial<Record<ExtendableInterfaces, Extension>>} Extensions */
+
+/**
+ * Parser options.
+ * @typedef {Object} ParserOptions
+ * @property {string} [sourceName]
+ * @property {boolean} [concrete]
+ * @property {Function[]} [productions]
+ * @property {Extensions} [extensions]
+ */
+
 /**
  * @param {Tokeniser} tokeniser
- * @param {object} options
- * @param {boolean} [options.concrete]
- * @param {Function[]} [options.productions]
+ * @param {ParserOptions} options
  */
 function parseByTokens(tokeniser, options) {
   const source = tokeniser.source;
@@ -33,7 +44,9 @@ function parseByTokens(tokeniser, options) {
     const callback = consume("callback");
     if (!callback) return;
     if (tokeniser.probe("interface")) {
-      return CallbackInterface.parse(tokeniser, callback);
+      return CallbackInterface.parse(tokeniser, callback, {
+        ...options?.extensions?.callbackInterface,
+      });
     }
     return CallbackFunction.parse(tokeniser, callback);
   }
@@ -41,20 +54,32 @@ function parseByTokens(tokeniser, options) {
   function interface_(opts) {
     const base = consume("interface");
     if (!base) return;
-    const ret =
-      Mixin.parse(tokeniser, base, opts) ||
-      Interface.parse(tokeniser, base, opts) ||
-      error("Interface has no proper body");
-    return ret;
+    return (
+      Mixin.parse(tokeniser, base, {
+        ...opts,
+        ...options?.extensions?.mixin,
+      }) ||
+      Interface.parse(tokeniser, base, {
+        ...opts,
+        ...options?.extensions?.interface,
+      }) ||
+      error("Interface has no proper body")
+    );
   }
 
   function partial() {
     const partial = consume("partial");
     if (!partial) return;
     return (
-      Dictionary.parse(tokeniser, { partial }) ||
+      Dictionary.parse(tokeniser, {
+        partial,
+        ...options?.extensions?.dictionary,
+      }) ||
       interface_({ partial }) ||
-      Namespace.parse(tokeniser, { partial }) ||
+      Namespace.parse(tokeniser, {
+        partial,
+        ...options?.extensions?.namespace,
+      }) ||
       error("Partial doesn't apply to anything")
     );
   }
@@ -73,11 +98,11 @@ function parseByTokens(tokeniser, options) {
       callback() ||
       interface_() ||
       partial() ||
-      Dictionary.parse(tokeniser) ||
+      Dictionary.parse(tokeniser, options?.extensions?.dictionary) ||
       Enum.parse(tokeniser) ||
       Typedef.parse(tokeniser) ||
       Includes.parse(tokeniser) ||
-      Namespace.parse(tokeniser)
+      Namespace.parse(tokeniser, options?.extensions?.namespace)
     );
   }
 
@@ -100,6 +125,7 @@ function parseByTokens(tokeniser, options) {
     }
     return defs;
   }
+
   const res = definitions();
   if (tokeniser.position < source.length) error("Unrecognised tokens");
   return res;
@@ -107,11 +133,7 @@ function parseByTokens(tokeniser, options) {
 
 /**
  * @param {string} str
- * @param {object} [options]
- * @param {*} [options.sourceName]
- * @param {boolean} [options.concrete]
- * @param {Function[]} [options.productions]
- * @return {import("./productions/base.js").Base[]}
+ * @param {ParserOptions} [options]
  */
 export function parse(str, options = {}) {
   const tokeniser = new Tokeniser(str);
diff --git a/test/custom-production.js b/test/custom-production.js
index 245fdec6..7c11dce7 100644
--- a/test/custom-production.js
+++ b/test/custom-production.js
@@ -1,51 +1,87 @@
-"use strict";
-
 import { expect } from "expect";
-import { parse, write } from "webidl2";
 
-describe("Writer template functions", () => {
-  const customIdl = `
-    interface X {};
-    custom Y;
-  `;
+import { parse, write } from "webidl2";
+import { Base } from "../lib/productions/base.js";
+import {
+  autoParenter,
+  type_with_extended_attributes,
+} from "../lib/productions/helpers.js";
 
-  /**
-   * @param {import("../lib/tokeniser").Tokeniser} tokeniser
-   */
-  const customProduction = (tokeniser) => {
-    const { position } = tokeniser;
-    const base = tokeniser.consumeIdentifier("custom");
-    if (!base) {
-      return;
-    }
-    const tokens = { base };
-    tokens.name = tokeniser.consumeKind("identifier");
-    tokens.termination = tokeniser.consume(";");
-    if (!tokens.name || !tokens.termination) {
-      tokeniser.unconsume(position);
+class CustomAttribute extends Base {
+  static parse(tokeniser) {
+    const start_position = tokeniser.position;
+    const tokens = {};
+    const ret = autoParenter(
+      new CustomAttribute({ source: tokeniser.source, tokens })
+    );
+    tokens.base = tokeniser.consumeIdentifier("custom");
+    if (!tokens.base) {
+      tokeniser.unconsume(start_position);
       return;
     }
-    return {
-      type: "custom",
-      tokens,
-      /** @param {import("../lib/writer.js").Writer} w */
-      write(w) {
-        return w.ts.wrap([
-          w.token(this.tokens.base),
-          w.token(this.tokens.name),
-          w.token(this.tokens.termination),
-        ]);
-      },
-    };
-  };
+    ret.idlType =
+      type_with_extended_attributes(tokeniser, "attribute-type") ||
+      tokeniser.error("Attribute lacks a type");
+    tokens.name =
+      tokeniser.consumeKind("identifier") ||
+      tokeniser.error("Attribute lacks a name");
+    tokens.termination =
+      tokeniser.consume(";") ||
+      tokeniser.error("Unterminated attribute, expected `;`");
+    return ret.this;
+  }
 
-  const result = parse(customIdl, {
-    productions: [customProduction],
-    concrete: true,
+  get type() {
+    return "custom attribute";
+  }
+
+  write(w) {
+    const { parent } = this;
+    return w.ts.definition(
+      w.ts.wrap([
+        this.extAttrs.write(w),
+        w.token(this.tokens.base),
+        w.ts.type(this.idlType.write(w)),
+        w.name_token(this.tokens.name, { data: this, parent }),
+        w.token(this.tokens.termination),
+      ]),
+      { data: this, parent }
+    );
+  }
+}
+
+describe("Parse IDLs using custom productions", () => {
+  it("Parse and rewrite top-level custom attribute", () => {
+    const customIdl = "custom long bar;";
+    const result = parse(customIdl, {
+      productions: [CustomAttribute.parse],
+      concrete: true,
+    });
+    expect(result[0].type).toBe("custom attribute");
+
+    const rewritten = write(result);
+    expect(rewritten).toBe(customIdl);
   });
-  expect(result[0].type).toBe("interface");
-  expect(result[1].type).toBe("custom");
+});
 
-  const rewritten = write(result);
-  expect(rewritten).toBe(customIdl);
+describe("Parse IDLs using custom extensions", () => {
+  [
+    ["callback interface", "callbackInterface"],
+    ["dictionary", "dictionary"],
+    ["interface", "interface"],
+    ["interface mixin", "mixin"],
+    ["namespace", "namespace"],
+  ].forEach(([type, key]) => {
+    it(`Attribute on ${type}`, () => {
+      const customIdl = `${type} Foo {
+        custom long bar;
+      };`;
+      const result = parse(customIdl, {
+        concrete: true,
+        extensions: { [key]: { extMembers: [[CustomAttribute.parse]] } },
+      });
+      expect(result[0].type).toBe(type);
+      expect(result[0].members[0].type).toBe("custom attribute");
+    });
+  });
 });