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Re: [comment-rex] XPath interoperability

From: Robin Berjon <robin.berjon@expway.fr>
Date: Mon, 9 Oct 2006 14:12:56 +0200
Message-Id: <E31EB3D0-BD40-4C6E-BC65-E6A493D29C74@expway.fr>
Cc: public-webapi@w3.org
To: karl@w3.org

Hi Karl,

On Jul 03, 2006, at 08:32, karl@w3.org wrote:
> [[[User-agents MAY support a superset of this syntax so long as it  
> is a valid instance of the XPath language [XPATH]. Content  
> producers however SHOULD NOT use such extensions as they hamper  
> interoperability.]]]
>
> This is slippery (specifically when you are hunting dahut ;). If  
> one user agent proposes the feature at a point in time where it  
> dominates the market, everyone will use it. Another risk of user  
> agent sniffing to send the right rex to the right user agent.

Things are always slippery when you're hunting dahut, that's half the  
fun!

We are well-aware of what might happen if implementations start using  
a larger subset, but equally we know that putting "don't do that" in  
the specification is not going to change what will happen. This  
clause is basically stating that it's a bad idea, while recognising  
what will happen in reality.

> What's happening when a author used the extended possibilities of a  
> browser X but they are not supported in the browser Y?

The path is considered invalid in UA Y, and therefore returns an  
empty node-set.

> Request a warning mechanism for user agent to inform users that the  
> rex message contains XPath features which are not processable.

The specification deliberately does not require UAs to inform the end- 
user of errors, as that tends to either be impractical, or simply not  
be implemented. They are however allowed to, and the specification  
does not preclude lint-like products.

> A specific event could be sent by the browser so it will be machine  
> identifiable as well.

We considered this but have so far deferred it to a future version.  
The reason for this is that v1 needs to be limited and simple, and  
because there have not been requirements for this feature.

Thanks again!

-- 
Robin Berjon
    Senior Research Scientist
    Expway, http://expway.com/
Received on Monday, 9 October 2006 12:13:06 GMT

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