> On Jan 13, 2015, at 10:22 AM, Boris Zbarsky <bzbar...@mit.edu> wrote: > >> On 1/13/15 1:18 PM, Ryosuke Niwa wrote: >> I agree. It's unusual for a constructor of a super class to automatically >> instantiate an arbitrary subclass based on its arguments. And we usually >> solve that convenience problem by introducing a factory class/function. > > While true, I do think there's a problem here. Consider this: > > var element = new HTMLElement("somename"); > > OK, so the web author is not being very forward-compatible in that they're > not using a tag name with a "-" in it. But then they put it in the DOM and > it acts just like a span, and they're happy with that.
Shouldn't we throw in this case because the concert type of "somename" is HTMLUnknownElement? > Then we want to add a "somename" tag in the spec, and suddenly this JS > throws. This is a different order of breakage than what you get from just > having new semantics for the "somename" tag. ... > In any case, it's a bit of a niggling worry for me because it can increase > the chance that adding things to HTML breaks websites. I think if we threw an exception on every attempt to create an element with a name without "-" (as they're HTMLUnknownElement anyway), then we can probably mitigate this forward compatibility issue. Hopefully, authors won't be creating HTMLUnknownElement all that often... - R. Niwa