I created an issue specifically for the BroadcastChannel discussion:
https://github.com/WICG/nav-speculation/issues/141

We'd love to hear if there are other issues with restrictions (or other) 
that need addressing!
Browse/open at https://github.com/WICG/nav-speculation/issues

On Monday, February 14, 2022 at 6:38:40 PM UTC+2 [email protected] wrote:

> On Mon, Feb 14, 2022 at 11:33 AM Noam Rosenthal <[email protected]> 
> wrote:
>
>>
>>
>> On Saturday, February 12, 2022 at 7:11:29 AM UTC+2 [email protected] 
>> wrote:
>>
>>> I agree with Domenic that it's great to see this kind of feature, that 
>>> was traditionally unspecified, getting some clearer developer visibility 
>>> and a spec. While there may still be missing pieces, this seems like a good 
>>> start. I'm looking forward to further spec discussions that would hopefully 
>>> lead to convergence and better interop.  
>>>
>>> On Fri, Feb 11, 2022 at 11:39 PM Domenic Denicola <[email protected]> 
>>> wrote:
>>>
>>>>
>>>>
>>>> On Fri, Feb 11, 2022 at 11:53 AM Olli Pettay <[email protected]> 
>>>> wrote:
>>>>
>>>>> FWIW, the comment in the HTML spec triage was positive feedback to 
>>>>> have a spec for this.
>>>>> The current spec proposal clearly is missing still quite a few cases 
>>>>> (is the idea really that one can use BroadcastChannel with prerendered 
>>>>> page? and the webidl annotation behaves rather oddly)
>>>>> So it is surprising to see this shipping already now. 
>>>>>
>>>>
>>>> Thanks for chiming in, Olli!
>>>>
>>>> I have a rather different take. As the team's spec mentor, I'm 
>>>> impressed with the spec work the team has done, on taking what most other 
>>>> browsers have treated as a not-to-be-specified user agent UI feature, and 
>>>> turning it into something much more rigorous and developer-friendly. For 
>>>> example:
>>>>
>>>>    - The careful auditing of all APIs to see how they should behave in 
>>>>    prerendering.
>>>>    - Introducing a well-specified Sec-Purpose: prefetch header, 
>>>>    instead of the unspecified X-moz: prefetch or X-Purpose: preview 
>>>> headers.
>>>>    - Considering how to handle edge cases such as redirects, 204s, 
>>>>    Content-Disposition: attachment, or pages that do client-side 
>>>> navigation 
>>>>    while being prerendered.
>>>>
>>>> I think there's definitely still work to be done, as we try to move 
>>>> from the current world where every browser does something different into 
>>>> one that's more fully predictable for web developers. But I think this is 
>>>> similar to other features, like bfcache, where for many years the 
>>>> heuristics and rules used were unspecified (e.g. Cache-Control, unload 
>>>> handlers, BroadcastChannel), and then we've started a slow convergence 
>>>> process as browsers start to care more about interoperability.
>>>>
>>>> We can definitely tweak things, like Olli's example of 
>>>> BroadcastChannel, over time and as other browsers indicate willingness to 
>>>> converge. 
>>>>
>>>
>>> One potential problem with that approach is that sites may grow to rely 
>>> on that existence of e.g. BroadcastChannel and may break when we take that 
>>> away.
>>> I don't think that's a risk for the current I2S, as developers are not 
>>> aware that the page is being prerendered outside of the page itself, so the 
>>> chances of them trying to communicate with it are low.
>>> But that can be a risk for the speculation rules based API, which would 
>>> be good to mitigate.
>>>
>>>>
>>>> Re. BroadcastChannel and friends - it's a totally valid point and 
>> there's an open issue about it (
>> https://github.com/WICG/nav-speculation/issues/113). I feel that we 
>> should examine these especially as we move from omnibox to 
>> document-initiated prerenering.
>>
>
> Yeah, just to be clear, I agree page-initiated prerendering multiplies the 
> risks significantly and will thus need more work.  I expect there will 
> still be some implementation-dependent behavior (e.g., some implementations 
> may discard a prerender if they use a disruptive API, while others might 
> delay the API results). But things like "does BroadcastChannel work at all 
> or not" are critical to nail down for interoperable page-initiated 
> prerendering.
>  
>

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