Custom elements: built-in element extensions lose their is attribute

[barneycarroll]

As demonstrated here, a custom element that extends a built-in via the is attribute behaves strangely. The extension is recognised and the custom element is constructed as expected; but the is attribute that determined this behaviour is not present on the rendered element. Having been specified in hyperscript, I would expect it to remain.

[pygy]

As long as we deal with [is], we have a render bug in the update phase. is should be considered part of the element identity along with the tag name when diffing.

[kfule]

When creating elements in Javascript, “is” may need to be given not only with createElement() but also explicitly with setAttribute().

flems (same issue without mithril)

As for mithril, it may be enough to just remove the "is" exclusion below and let setAttr() work like any other attribute.

mithril.js/render/render.js

Line 679 in c6ffa18

if (key === "key" || key === "is" || value == null || isLifecycleMethod(key) || (old === value && !isFormAttribute(vnode, key)) && typeof value !== "object") return