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Re: The implied @about="": Explanation and some problems

From: Richard Cyganiak <richard@cyganiak.de>
Date: Wed, 1 Apr 2009 14:13:46 +0100
Cc: RDFa <public-rdf-in-xhtml-tf@w3.org>, "public-rdfa@w3.org" <public-rdfa@w3.org>
Message-Id: <F145F8A4-FFF8-4DDB-8FB4-C7485AE50C75@cyganiak.de>
To: Mark Birbeck <mark.birbeck@webbackplane.com>
On 1 Apr 2009, at 10:15, Mark Birbeck wrote:
> Now, although the principle seems sound, the rules defined in order to
> achieve it might be causing some problems. The first is one that I
> think was flagged up by Ivan a while ago, but I'll list it here to jog
> your memories; if you put a subject onto the root (most likely the
> HTML element), then your subject gets overridden when parsing hits the
> <head> and <body>:
>  <html about="http://somewhereelse.com/">
>    ...
>  </html>
> Perhaps we could just live with that, but advise people that if they
> want to do this they should really be using <base>. But either way,
> it's still a quirk.

I think this issue is a bit deeper than you make it sound. And using  
<base> doesn't always address the issue. Actually, I find that <base>  
and RDFa can mix in an unfortunate way.

It's fairly common practice among web developers to set <base> to the  
homepage of the site on all the site's pages. This makes it easier to  
manage relative links throughout the sites, as all relative URIs in  
the site can be done relative to the homepage.

But this has the effect that in RDFa, the subject no longer defaults  
to the URI of the *page*, but to the URI of *another page*, which is  
usually not what I want (I want my @property="dc:title" and  
@rel="foaf:primaryTopic" to apply to the individual page, not to the  

So I need to override the <base> with @about in order to have the  
desired default subject in RDFa. But because the implicit @about="" is  
added *twice* (at <head> and <body>), I also need to override it  

I think my point is that

a) I don't get why the invisible @about="" is on <head>+<body> instead  
of on <html> -- can someone explain?

b) I want to call attention to the fact that specifying <base> can  
wreak havoc on a page's RDFa, because obvious idioms such as  
@property="dc:title" on the <title> element suddenly don't work any  


> The second issue is the use of @typeof on <body> or <head>; I have a
> vague recollection this also came up in Ivan's example, but I might be
> wrong, but either way, it also came up for me today when I was asked
> to check someone's RDFa documents. They have this in their document:
>  <body typeof="foaf:Document">
>    ...
>  </body>
> My parser gave this triple:
>  <> a <http://xmlns.com/foaf/0.1/Document> .
> and since I was expecting the subject to be a bnode, I assumed there
> was a bug in my parser. However, looking at the spec I see that we do
> indeed place the 'implied @about' at a higher level than the bnode:
>  4. If the [current element] contains no @rel or @rev attribute,  
> then the next
>      step is to establish a value for [new subject]. Any of the
> attributes that can
>      carry a resource can set [new subject];
>      [new subject] is set to the URI obtained from the first match
> from the following
>      rules:
>        @about...@src...@resource...@href, etc.;
>      If no URI is provided by a resource attribute, then the first
> match from the
>      following rules will apply:
>        if the element is the head or body element then act as if
> there is an empty
>        @about present, and process it according to the rule for  
> @about, above;
>        if @typeof is present, obtained according to the section on
>        Processing, then [new subject] is set to be a newly created  
> [bnode].
>        otherwise, if [parent object] is present, [new subject] is set
> to the value of
>        [parent object]. Additionally, if @property is not present
> then the [skip
>        element] flag is set to 'true';
> It's these last three rules that we're focusing on.
> I would argue that since the intention of setting @about="" on
> head/body was simply to 'tidy up' the initial subject so that it
> didn't have any fragment identifiers, then the rule that achieves this
> should only be applied if the subject wasn't set in any other way.
> This would be easily achieved if we moved the rule to the end of the
> group of three quoted above. That would solve both problems mentioned
> at the top, because:
>  * if @typeof is used on <head> or <body> then a bnode is created,
> making it consistent
>    with processing in other situations;
>  * if there is a parent subject (i.e., on <html>) then that is used,
> and no 'implied @about'
>    is needed.
> You could argue that this then removes an easy way to indicate the
> type of the *document*, but I think the answer to that is that the
> reordered rules would simply force you to be explicit; if you want to
> set the type of the document rather than generating a bnode, then you
> would simply do this:
>  <body about="" typeof="foaf:Document">
>    ...
>  </body>
> I realise changing the spec or issuing an errata is not something that
> can be taken lightly, so this email is primarily about completing my
> action item to explain what the implied @about was all about.
> Then I suppose the next step would be to see whether we just live with
> the quirks that we have, or whether we want to tidy them up. And if
> so, we should probably try to find out if anyone is actually producing
> documents with @typeof on <body> or <head>, and if they are, what's
> the effect they are trying to achieve.
> Regards,
> Mark
> -- 
> Mark Birbeck, webBackplane
> mark.birbeck@webBackplane.com
> http://webBackplane.com/mark-birbeck
> webBackplane is a trading name of Backplane Ltd. (company number
> 05972288, registered office: 2nd Floor, 69/85 Tabernacle Street,
> London, EC2A 4RR)
Received on Wednesday, 1 April 2009 13:14:29 UTC

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