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I think we should consider dropping iframe@seamless from the spec due to lack of implementer interest. Among the indicators that I think suggest there's not enough interest to justify keeping it in the spec is that Blink has dropped the partial (and unshipped in Chrome) implementation of it that was inherited from the WebKit days. https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/8tcNMmWJEwQ/N0WBYAC_tZMJ https://groups.google.com/a/chromium.org/d/msg/blink-dev/8tcNMmWJEwQ/UQXDdNFDaHcJ The stated reasons include that not even product teams like Gmail and Google+ were interested in deploying and using it even if it was fully implemented, and that it introduced complexity costs for which the benefits (in light of the lack of deployment interest) were viewed as being too small the costs. Along with that, I don't think any other browser projects so far have shown any real interest in implementing seamless. As a data point, note that I raised a Gecko implementation bug for seamless two years ago, but during the two years since I raised it the bug has had had zero comments from any Mozilla implementers. https://bugzilla.mozilla.org/show_bug.cgi?id=631218
I agree that it's a low-priority feature for browser vendors today, but I disagree that it has had no browser interest (I mean, it was designed by Gecko and WebKit (now Blink) engineers, mostly), and it has a lot of author interest (we get requests to expand it to include cross-origin seamless stuff regularly, to the point where we have a bug on it now: bug 23513).
I guess for now I'm going to decide to keep it in. If there's still no implementation interest after a few years, then we should revisit this, obviously. But this feature was developed with browser vendors, so there's clearly _some_ interest, even if it's no longer a high priority. Many features in specs have taken years to go from design to implementation, it doesn't mean we should drop them, IMHO.
WFM. I just raised it to get your consideration of it, and your assessment makes sense to me.
For the historical record here, iframe[seamless] did end up being dropped from HTML: https://github.com/whatwg/html/commit/1490eba4dba5ab476f0981443a86c01acae01311