The Web is my API

Michael Mahemoff taught me a lot about the possibilities of the web. Prior to working with Mike I built on the web and I understood the benefits such as linkability and discovery, but I never really had a full picture of what would be possible.

One thing that Mike said was "the Web is my API", where he talked about the being able to expose your site and your data in a page via microformats and other structured data and being able to access it directly from another another browser context, using a simple XMLHttpRequest and the CORS API:

Anyway, what’s cool about this is you can treat the web as an API. The Web is my API. "Scraping a web page" may sound dirtier than "consuming a web service", but it’s the cleaner approach in principle. A website sitting in your browser is a perfectly human-readable depiction of a resource your program can get hold of, so it’s an API that’s self-documenting. The best kind of API. But a whole HTML document is a lot to chew on, so we need to make sure it’s structured nicely, and that’s where microformats come in, gloriously defining lightweight standards for declaring info in your web page. There’s another HTML5 tie-in here, because we now have a similar concept in the standard, microdata.

It was around the same time that I was starting to work on Web Intents, the spirit of which was the similar — give users access to data and services from another origin — but it was a lot more complex. I wanted to enable discovery of services and then to interact with those pages. And Mike wanted to move the web to providing access to data and services. It stuck with me. Even if I did forget the original attribution.

I recently did a talk for Nordic JS where I highlighted that we don't build really build truley interconnected services on the web, and when we do it follows a model of mostly server to server interactions. That is a web site will integrate with a 3rd party service by routing all API request through their server to the remote service and managing all the complexities that come with that.

It works, we have entire web built with this, but it can be incredibly complex when you consider authenticaion, authorization, transport protocols and differing RPC methods (REST, GraphQL etc). Mike was proposing something much more elegant, that with CORS enabled sites and a bit of JavaScript, we can talk directly to the remote service by using the site.

There have been a couple of issues that cropped up in between. The primary issue is that even though CORS is widely supported in browsers, developers rarely use it. CORS is a protection that we need on the web but it's hard to set up and debug, and the "Web as an API" has not really been pushed too much.

We are moving to a world where sites are getting generated in the client with JS and sessions and state for the user are managed in the entirely on the client.

We still need the ability to communicate from our sites to a remote service, and I still strongly believe that we need to decentralize our integrations with other sites and apps, but the first thing that we need to do is connect our sites and apps together in away that is more than just a link. We need our sites to expose their capabilities and functionality directly to other windows on the users system.

Every website should be able to expose an API that the owner of the site is control of, directly to other clients.

The good news is that we can already do it, we've had the primitives on the platform for at least 7 years (postMessage and MessageChannel), and forever since, but we don't use these tools to interact with sites for similar reasons why we don't use CORS: It's hard and it's nearly impossible to define a sane API that is simple and consistent to use and that doesn't require pulling in huge third party libraries for each service that you want to interact with.

The platform only allows you to communicate between sites using messaging passing which means that as a service owner if you wanted to create an API, you have to build a state machine that serializes messages in to some state, reacts to it, and then sends a message back to the client and then you have to create a library that does that for the developer consuming your service. It's incredibly complex and convoluted and I belive is one of the primary reasons why we haven't seen more adoption of Web Workers and client-side APIs.s

We have a library that helps: Comlink.

Comlink is a small API that abstracts the MessageChannel and postMessage API's in to an API that looks like you are instantiating remote classes and functions in the local context. For example:


// Set up.
const worker ='somesite');
const api = Comlink.proxy(w);

// Use the API.
const work = await new api.Test();
const str = await work.say('Yo!');

Web Worker

class Test {
  say() {
    return `Hi ${this.x++}, ${msg}`;

// Expose the API to anyone who connects.
Comlink.expose({Test}, window);

We expose an API on the service, we consume the API in the client via a proxy.

Is there a better example?

I built a site that subscribes to a pubsubhubbub endpoint and when it recieves a ping it sends a JSON message to a user defined endpoint. I didn't want to manage the push notification infrastructurion for this small app, another site I built ( can do all that, I just want to use integrate with that service..

But how do I get the subscription URL (the piece of data I need to be able to send notifications) held in the client of back into my site?

When I initially built this site, all you could do was let the user open the site and then copy and paste the URL between the pages. Why not just expose an API that any site could use? That's what I did. defines an API called PushManager that has a single method on it subscriptionId. When the page loads it exposes this API to the window as follows:

class PushManager {
  constructor() {

  async subscriptionId() {
    //global var ick...
    let reg = await navigator.serviceWorker.getRegistration();
    let sub = await reg.pushManager.getSubscription();
    if(sub) {
        return `${location.origin}/send?id=${sub.endpoint}`;
    else {
        return ``;

Comlink.expose({PushManager}, window);

The API interacts with the PushSubscriptionManager API in the DOM and returns a custom URL to the calling site. The important thing here is that because it is running asynchronously I can wait for user verifcation that they want to perform the action (or not).

Back on the calling client site (the app that wants to get the subscriptionId). When a user clicks on the link, we obtain a reference to the window we just opened and hook-up our Comlink proxy to it. The service API is now exposed to our client and we can instantiate the PushManager API like it was a local service, but it is all interacting with the remote instance service in the other window.

let endpointWindow ='', 'endpointUrlWindow');

let pushAPI = Comlink.proxy(endpointWindow);
let pm = await new pushAPI.PushManager();
let id = await pm.subscriptionId();

// Update the UI.
endpointUrlEl.value = id;

Here is a very quick video of what is happening. A very simple and light-weight interaction, it opens the service and then gets the ID it needs.

As a service provider I have exposed a constrained set of functionality that is only available on the client to another site and I can secure it and ask for user consent at the same time before I return the data back to the user, all with a simple to use API.

The Web is the API.

Quite rightly, we don't let sites inspect or manipulate the DOM or state of another origin, but I posit that if you have control over the services and functionality of your site and how users engage with it then you can expose the most important information and services to any client that wants to use your service securely (you are in control) and it allows you to:

What API's should sites expose?

This is something that I would like to explore more. I exposed some basic functionality to a Push Notifications service because that is the intent of the site, but the important piece for me was that I was in control of which parts of the DOM that I wanted to give back to other developers.

I would like to get to a place where every site can expose a consistent API to users and a way to discover other functionality and services.

Each site owner could expose just the core functionality to their service so that we could do CRUD based operations. We could have complex interactions.

We could get to a web where we have Unix like services that do one thing well and a user pipes them together all on the client.

Each site could expose a VDOM of a subset of the page that is defined by the service owner so that we have a consistent way to pull move data based on the DOM between sites securely.

I could imagine that we might want quick access to all of the based objects or other structured data on the page (they could be dynamically generated) like Mike did in his original post.

Comlink gives us a way to expose and consume services quickly and easily on top of the platform primitives that have been around for years. We finally have a lot of the pieces in place that allow us to make the Web the API.

The Web is my API. Make it yours too.

I lead the Chrome Developer Relations team at Google.

We want people to have the best experience possible on the web without having to install a native app or produce content in a walled garden.

Our team tries to make it easier for developers to build on the web by supporting every Chrome release, creating great content to support developers on, contributing to MDN, helping to improve browser compatibility, and some of the best developer tools like Lighthouse, Workbox, Squoosh to name just a few.

I love to learn about what you are building, and how I can help with Chrome or Web development in general, so if you want to chat with me directly, please feel free to book a consultation.

I'm trialing a newsletter, you can subscribe below (thank you!)