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Archive for the ‘WHATWG’ Category

Working mode

Tuesday, April 25th, 2017

In a previous post we’ve already explained how interoperability is important to the WHATWG. Without it, we’re writing fiction, and in the world of standards that is no good.

From a similar perspective, we’ve now more clearly documented how the WHATWG creates standards. The Working Mode document describes what is expected of editors and contributors, what criteria any changes to standards must fulfill, and gives guidelines for conflicts and tests.

What has changed the most since 2004 is requiring tests and implementer support for any changes made. These should help ensure that decisions need not be revisited again. Documenting our processes is also new and is born out of necessity due to the wider range of standards the WHATWG maintains.

We appreciate any feedback on the Working Mode document as it can undoubtedly be refined further.

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Improving interoperability

Monday, January 30th, 2017

The goal of the WHATWG’s Living Standards is to achieve interoperable implementations. With an ever-evolving web platform, we want changes to our standards to reach all implementations quickly and reliably, but from time to time there have been mishaps:

Three months ago, we changed the process for the HTML Standard to encourage writing tests and filing browser bugs for normative changes. (Normative means that implementations are affected.) This was the first step on a path towards improving interoperability and shortening the feedback cycle, and it has thus far exceeded our own expectations:

As an example, see Remove "compatibility caseless" matching where 3 of the 4 browser bugs are now fixed, or Add <script nomodule> to prevent script evaluation where all vendors have indicated support, and WebKit has a patch to implement the proposed feature and are contributing their tests to web-platform-tests—even before the standard’s pull request has landed.

Note in particular that this has not amounted to WHATWG maintainers writing all new tests. Rather, we are a community of maintainers, implementers and other contributors, where tests can be written to investigate current behavior before even discussing a change to the standard, or where the most eager implementer writes tests alongside the implementation.

We have been using this process successfully for other WHATWG standards too, such as Fetch, URL, and Streams. And today, we are elevating this process to all WHATWG standards, as now documented in the WHATWG contributor guidelines.

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Wednesday, November 16th, 2016

Welcome to the newest standard maintained by the WHATWG: the Infra Standard! Standards such as DOM, Fetch, HTML, and URL have a lot of common low-level infrastructure and primitives. As we go about defining things in more detail we realized it would be useful to gather all the low-level functionality and put it one place. Infra seemed like a good name as it’s short for infrastructure but also means below in Latin, which is exactly where it sits relative to the other work we do.

In the long term this should help align standards in their vocabulary, make standards more precise, and also shorten them as their fundamentals are now centrally defined. Hopefully this will also make it easier to define new standards as common operations such as “ASCII lowercase” and data structures such as maps and sets no longer need to be defined. They can simply be referenced from the Infra Standard.

We would love your help improving the Infra Standard on GitHub. What language can further be deduplicated? What is common boilerplate in standards that needs to be made consistent and shared? What data types are missing? Please don’t hesitate to file an issue or write a pull request!

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Sunsetting the JavaScript Standard

Monday, August 15th, 2016

Back in 2012, the WHATWG set out to document the differences between the ECMAScript 5.1 specification and the compatibility and interoperability requirements for ECMAScript implementations in web browsers.

A specification draft was first published under the name of “Web ECMAScript”, but later renamed to just “JavaScript”. As such, the JavaScript Standard was born.

Our work on the JavaScript Standard consisted of three tasks:

  1. figuring out implementation differences for various non-standard features;
  2. filing browser bugs to get implementations to converge;
  3. and finally writing specification text for the common or most sensible behavior, hoping it would one day be upstreamed to ECMAScript.

That day has come.

Some remaining web compatibility issues are tracked in the repository for the ECMAScript spec, which now redirects to. The rest of the contents of the JavaScript Standard have been upstreamed into ECMAScript, Annex B.

This is good news for everyone. Thanks to the JavaScript Standard, browser behavior has converged, increasing interoperability; non-standard features got well-defined and standardized; and the ECMAScript standard more closely matches reality.


So long, JavaScript Standard, and thanks for all the fish!

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Defining the WindowProxy, Window, and Location objects

Thursday, May 12th, 2016

The HTML Standard defines how navigation works inside a browser tab, how JavaScript executes, what the overarching web security model is, and how all these intertwine and work together. Over the last decade, we’ve made immense progress in specifying previously-unspecified behavior, reverse-engineering and precisely documenting the de-facto requirements for a web-compatible browser. Nevertheless, there are still some corners of the web that are underspecified—sometimes because we haven’t yet discovered the incompatibility, and sometimes because specifying the behavior in a way that is acceptable to all implementers is really, really hard. What follows is an account of the latter.

Until recently, the HTML Standard lacked a precise definition of the WindowProxy, Window, and Location objects. As you might imagine, these are fairly important objects, so having them be underdefined was not great for the web. (Note that the global object used for documents is the Window object, though due the way browsers evolved it is never directly exposed. Instead, JavaScript code accesses the WindowProxy object, which serves as a proxy and security boundary for the Window object.)

Each navigable frame (top-level tab, <iframe> element, et cetera) is called a browsing context in the HTML Standard. A browsing context has an associated WindowProxy and Window object. As you navigate a browsing context, the associated Window object changes. But the whole time, the WindowProxy object stays the same. Ergo, one WindowProxy object is a proxy for many Window objects.

To make matters more interesting, scripts in these different browsing contexts can access each other, through frame.contentWindow, self.opener,, et cetera. The same-origin policy generally forbids code from one origin from accessing code from a different origin, which prevents from prying into The two legacy exceptions to this rule are the WindowProxy and Location objects, which have some properties that can be accessed across origins.

document.domain makes this even trickier, as it effectively allows you to observe a WindowProxy or Location object as cross-origin initially, and same-origin later, or vice versa. Since the object remains the same during that time, the same-origin versus cross-origin logic needs to be part of the same object and cannot be spread across different classes.

As JavaScript has many ways to inspect objects, WindowProxy and Location objects were forced to be exotic objects and defined in terms of JavaScript’s “meta-object protocol”. This means they have custom internal methods (such as [[Get]] or [[DefineOwnProperty]]) that define how they respond to the low-level operations that govern JavaScript execution.

Defining this all in detail has been a multi-year effort spearheaded by Bobby Holley, Boris Zbarsky, Ian Hickson, Adam Barth, Domenic Denicola, and Anne van Kesteren, and completed in the “define security around Window, WindowProxy, and Location objects properly” pull request. The basic setup we ended up with is that WindowProxy and Location objects have specific cross-origin branches in their internal method implementation. These take care to only expose specific properties, and even for those properties, generating specific accessor functions per origin. This ensures that cross-origin access is not inadvertently allowed through something like Object.getOwnPropertyDescriptor(otherWindowProxy, "window").get. After filtering, a WindowProxy object will forward to its Window object as appropriate, whereas a Location object simply gives access to its own properties.

Having these objects defined in detail will make it easier for implementations to refactor, and for new novel implementations like Servo to achieve web-compatibility. It will reduce debugging time for web developers after implementations have converged on the edge cases. And it drastically simplifies extending these objects, as well as placing new restrictions upon them, within this well-defined subsystem. Well-understood, stable foundations are the key to future extensions.

(Many thanks to Bobby Holley for his contributions to this post.)

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