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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  

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 UTC

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