There is an ongoing implementation of the custom elements API <https://www.w3.org/TR/custom-elements/> on bug 154907.
This bug is for tracking the implementation of the GTK+ specific parts of this specification (if any), for enabling it and for testing it.
I don't think there will be any port specific code needed other than port specific WebKit API to be exposed if any.
(In reply to comment #1)
> I don't think there will be any port specific code needed other than port
> specific WebKit API to be exposed if any.
On the GTK+ port currently running the tests inside fast/custom-elements currently gives the following:
Regressions: Unexpected text-only failures (11)
fast/custom-elements/Document-createElement.html [ Failure ]
fast/custom-elements/Document-defineElement.html [ Failure ]
fast/custom-elements/HTMLElement-constructor.html [ Failure ]
fast/custom-elements/defined-pseudo-class.html [ Failure ]
fast/custom-elements/parser/parser-constructs-custom-element-synchronously.html [ Failure ]
fast/custom-elements/parser/parser-constructs-custom-elements.html [ Failure ]
fast/custom-elements/parser/parser-fallsback-to-unknown-element.html [ Failure ]
fast/custom-elements/parser/parser-sets-attributes-and-children.html [ Failure ]
fast/custom-elements/parser/parser-uses-constructed-element.html [ Failure ]
fast/custom-elements/upgrading/Node-cloneNode.html [ Failure ]
fast/custom-elements/upgrading/upgrading-parser-created-element.html [ Failure ]
Regressions: Unexpected image-only failures (1)
fast/custom-elements/defined-rule.html [ ImageOnlyFailure ]
Regressions: Unexpected timeouts (4)
fast/custom-elements/attribute-changed-callback.html [ Timeout ]
fast/custom-elements/lifecycle-callback-timing.html [ Timeout ]
fast/custom-elements/parser/parser-constructs-custom-element-in-document-write.html [ Timeout ]
fast/custom-elements/parser/parser-uses-registry-of-owner-document.html [ Timeout ]
On r195087 <http://trac.webkit.org/r195087> you set the the fast/custom-elements as expected to fail for all ports but the Mac one. However, some tests were timing out (instead of failing), this caused unexpected failures on the bots.
So I skipped completely the fast/custom-elements tests on r198993 <http://trac.webkit.org/r198993> for the GTK+ port linking to this bug.
The EFL port did something similar in r198779 <http://trac.webkit.org/r198779>
All tests added in r206838 are failing. I'll skip these too.
Is there a build or runtime flag for custom elements?
I'm enabling this feature everywhere on https://bugs.webkit.org/show_bug.cgi?id=164242.
Fixed in https://bugs.webkit.org/show_bug.cgi?id=164242.