WebDriver update
from TPAC 2015

I came back from the TPAC (the W3C’s Technical Plenary/Advisory Committee meeting week) earlier this month, where I attended the Browser Tools- and Testing Working Group’s meetings on WebDriver.

Unlike previous meetings, this was the first time we had a reasonably up-to-date specification text to discuss. That was clearly not a bad idea to have because we were able to make some defining decisions on long-standing, controversial topics. This shows how important it is for assigned action items to be completed in time before a specification meeting, and to have someone with time dedicated to working on the spec.


The WG decided to punt the element visibility, or “displayedness” concept, to level 2 of the specification and in the meantime push for better visibility primitives in the platform. I’ve previously outlined in detail the reasons why it’s not just a bad idea—but impossible—for WebDriver to specify this concept. Instead we will provide a non-normative description of Selenium’s visibility atom in an appendix to give some level of consistency for implementors.

Fortunately Selenlium’s visibility approximation atom can be implemented entirely in content JavaScript, which means it can be provided in both client bindings and as extension commands.

This does not mean we are giving up on visibility. There is general agreement in the WG that it is a desirable feature, but since it’s impossible to define naked eye visibility using existing platform APIs we call upon other WGs to help outline this. Visibility of elements in viewport is not a primitive that naturally fits within the scope of WebDriver.

Our decision has implications for element interactability, which is used to determine if you can interact with an element. This previously relied on the element visibility algorithm, but as an alternative to the tree traversal visibility algorithm we dismissed, we are experimenting with a somewhat naïve hit-testing alternative that takes the centre coordinates of the portion of the element inside the viewport and calls elementsAtPoint, ignoring elements that are opaque.

Attributes and properties

We had previously decided to make two separate commands for getting attributes and properties. This was controversial because it deviates from the behaviour of Selenium’s getAttribute, that conflates the DOM concepts of attributes and properties.

Because the WG decided to stick with David Burns’s proposal on special-casing boolean attributes, the good news is that the Selenium behaviour can be emulated using WebDriver primitives.

In practice this means that when Get Element Attribute is called for an element that carries a boolean attribute, this will return a string "true", rather than the DOM attribute value which would normally be an empty string. We return a string so that dynamically typed programming languages can evaluate this into something truthful, and because there is a belief in the WG that an empty string return value for e.g. <input disabled>, would be confusing to users.

Because we don’t know which attributes are boolean attributes from the DOM’s point of view, it’s not the cleanest approach since it means we must maintain a hard-coded list in WebDriver. It will also arguably cause problems for custom elements, because it is not given that they mirror the default attribute values.

Test suite

One of the requirements for moving to REC is writing a decent test suite. WebDriver is in the fortunate position that it’s an evolution of existing implementations, each with their own body of tests, many of whom we can probably re-purpose. One of the challenges with the existing tests is that the harness does not easily allow for testing the lower level details of the protocol.

So far I have been able to make a start with merging Microsoft’s pending pull requests. Not all the tests merged match what the specification mandates any longer, but we decided to do this before any substantial harness work is done, to eliminate the need for Microsoft to maintain their own fork of Web Platform Tests.


Microsoft and Mozilla are both working on implementations, so there is a pressing need for a test suite that reflects the realities of the specification. Vital chapters, such as Element Retrieval and Interactions, are either undefined or in such a poor state that they should be considered unimplementable.

Despite these reservations, I’d say the WebDriver spec is in a better state than ever before. At TPAC we also had meetings about possible future extensions, including permissions and how WebDriver might help facilitate testing of WebBluetooth as well as other platform APIs.

The WG is concurrently pushing for WebDriver to be used in Web Platform Tests to automate the “non-automatable” test cases that require human interaction or privileged access. In fact, there’s an ongoing Quarter of Contribution project sponsored by Mozilla to work on facilitating WebDriver in a sort of “meta-circular” fashion, directly from testharness.js tests.

But more on that later. (-: