Where do you see that in the spec? That section in particular definitely does not support it.

"Any namespace-less attribute that is relevant to the element's functioning, as determined by the element's author, may be specified on an autonomous custom element, so long as the attribute name is XML-compatible and contains no uppercase ASCII letters. The exception is the is attribute, which must not be specified on an autonomous custom element (and which will have no effect if it is)."

My implementation wasn't doing this properly.

That's a conformance requirement for authors, telling them what they must write to have valid HTML. It doesn't affect implementations, which must process is attributes according to the normative parts of the spec's processing model.

If someone creates an element with createElement('a-a', {is: 'b-b'}) it should create an autonomous custom element with definition of 'a-a' with 'is' attribute set to 'b-b', right?

In this case my implementation was looking for a custom element definition with name 'b-b' and localName 'a-a', which doesn't exist.

> If someone creates an element with createElement('a-a', {is: 'b-b'}) it should create an autonomous custom element with definition of 'a-a' with 'is' attribute set to 'b-b', right?

Well, I'm not sure? What does the spec say? Let me walk you through the process of finding out. Note how the section you quoted is never involved as it does not give any normative algorithm steps for createElement.

1. We start at https://dom.spec.whatwg.org/#dom-document-createelement and go to step 5
2. That takes us to https://dom.spec.whatwg.org/#concept-create-element passing document, 'a-a', the HTML namespace, null, 'b-b', and with the synchronous custom elements flag set.
3. Step 4 takes us to https://html.spec.whatwg.org/multipage/scripting.html#look-up-a-custom-element-definition passing document, the HTML namespace, 'a-a', and 'b-b'
4. In this algorithm, does step 4's condition hold? Maybe, if previously 'a-a' has been defined as an autonomous custom element. Let's call that case (A).
5. If 'a-a' has not been defined, we get to step 5. Is there a custom element definition with name equal to 'b-b' and local name equal to 'a-a'? Maybe. Let's call that case (B).
6. Otherwise, if neither step 4 or step 5's condition hold, we return null. Let's call that case (C).

In case (A), following the logic in https://dom.spec.whatwg.org/#concept-create-element will take us to step 6.1, calling the constructor for a-a which executes the relevant constructor and sets the `is` value to null. Popping back to https://dom.spec.whatwg.org/#dom-document-createelement step 6, we'll set the `is` attribute.

In case (B), we'll end up at step 5, which sets the `is` value to 'b-b' and enqueues an upgrade. Before the upgrade happens, we'll also set the `is` attribute.

And in case (C), we'll end up in step 7, which creates a new undefined element, but sets its `is` value to 'b-b'. We'll also set its `is` attribute.

----

I'm not sure which of case (A), (B), or (C) you were referring to. But in genreal the `is` option will have an effect, definitely. You have to follow the algorithm steps to work through it.

I was referring to case (A), but my implementation wrongly responded with case (B)'s result. This is because I don't conform to the specs in https://dom.spec.whatwg.org/#dom-document-createelement. What I posted in the description above is the part of the spec that reminded me to test case (A) in the first place. I'll update the description to be more specific.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f7888049506013dd915e6d9cbd03bbe837ccf271

commit f7888049506013dd915e6d9cbd03bbe837ccf271
Author: yurak <yurak@google.com>
Date: Sun Nov 20 08:50:15 2016

Added some tests according to https://html.spec.whatwg.org/multipage/scripting.html#custom-elements-core-concepts

Fixed a bug where:
    The 'is' attribute must not be specified on an autonomous custom element, if it is it has no effect

BUG= 666166 

Review-Url: https://codereview.chromium.org/2502223002
Cr-Commit-Position: refs/heads/master@{#433451}

[modify] https://crrev.com/f7888049506013dd915e6d9cbd03bbe837ccf271/third_party/WebKit/LayoutTests/custom-elements/spec/create-element.html
[modify] https://crrev.com/f7888049506013dd915e6d9cbd03bbe837ccf271/third_party/WebKit/Source/core/dom/custom/CustomElementRegistry.cpp