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Re: ISSUE-117 (about-on-HTML): Consider disallowing @about on <html> [RDFa 1.1 in HTML5]

From: Ivan Herman <ivan@w3.org>
Date: Sun, 20 Nov 2011 13:17:15 +0100
Cc: Toby Inkster <tai@g5n.co.uk>, RDF Web Applications Working Group WG <public-rdfa-wg@w3.org>, sysbot+tracker@w3.org
Message-Id: <41501A66-9590-4C72-8645-A7D6EA648A05@w3.org>
To: Mark Birbeck <mark.birbeck@webbackplane.com>
Thanks Mark,

good to have you around:-)

The only minor issue in what you are saying is that we _do_ have a hidden default @about in <html>; indeed, the processing rules explicitly refer to the fact that the top element sets the subject to the document URI (unless explicitly said). Ie, we have some sort of a mixture here.

I am not sure whether you followed the mailing list. The problem is that when moving all this to HTML5, very unexpected things happen simply because HTML5 is permissive and allows for a head/body element to be missing in the original source, but will be added to the DOM tree. Toby, who is unbeatable in identifying such edge cases:-) has shown some horrible examples. On the other hands, the (purely anecdotical) experience shows that very few users (if any) used the trick of putting an empty @typeof into the <head> or the <body>. Hence the proposal to drop these (but keeping the default @about in <html>).


On Nov 20, 2011, at 12:59 , Mark Birbeck wrote:

> Hi Ivan,
> You wrote:
>> What this rule says, in terms of elements, is that <html> (or any top element) _has_ this magic
>> behaviour, ie, an @about="" is introduced on that level, conceptually (unless there is an explicit
>> @about, that is). So what does <head> and <body> magic brings us?
>> I am officially lost.:-)
> :)
> Bear with me, and I'll reconstruct the rationale, for you...
> As you know, part of the RDFa story was to provide an RDF
> interpretation of already existing HTML metadata features -- i.e., if
> an existing HTML document that had no RDFa in it was parsed with an
> RDFa parser, we would still like to end up with something sensible.
> Take an example from the LINK element section of the HTML 4 spec [1]:
>  <HEAD>
>    ...other head information...
>    <TITLE>Chapter 5</TITLE>
>    <LINK rel="prev" href="chapter4.html">
>    <LINK rel="next" href="chapter6.html">
>  </HEAD>
> It's obvious that the 'something sensible' that we would like to end
> up with is two triples that provide two predicates for the 'current
> document', one of which points to a document that is somehow 'prior'
> to the 'current document' and the other which is somehow 'next'.
> Expressed as triples we would want:
>  <> some-ns:prev <chapter4.html> .
>  <> some-ns:next <chapter6.html> .
> By adopting this approach we get to derive RDF from pre-existing HTML
> that has been created by authors that have no knowledge of (or
> interest in) RDF. A result. :)
> Now, let's add some RDFa to this document so that we can indicate that
> the 'current document' is not just any old document, but it's actually
> a book. This property of the document /could/ be indicated by adding
> @typeof to the root element (excuse me whilst I also switch to
> lower-case for element names!):
>  <html typeof="bibio:book">
>    <head>
>      <title>Chapter 5</title>
>      <link rel="prev" href="chapter4.html">
>      <link rel="next" href="chapter6.html">
>    </head>
>    .
>    .
>    .
> Unfortunately @typeof with no subject automatically creates a bnode
> which would mean that the markup inside <head> would no longer
> reference the 'current document' -- it would instead reference the
> newly minted bnode. Given that we wanted authors to feel their way
> through RDFa by incremental additions it would be quite a surprise for
> them if the simple addition of a type description for a document
> caused all of the previous predicates attached to that document to
> disappear.
> One way to get around this would have been to have put a default of
> @about="" on the HTML element, so that our previous example was parsed
> as if the following was present:
>  <html about="" typeof="bibio:book">
>    <head>
>      <title>Chapter 5</title>
>      <link rel="prev" href="chapter4.html">
>      <link rel="next" href="chapter6.html">
>    </head>
>    .
>    .
>    .
> However, this only moves the problem to the <head> element; if instead
> of the above pattern someone used the following one we'd once again
> have a bnode to contend with:
>  <head typeof="bibio:book">
>    <title>Chapter 5</title>
>    <link rel="prev" href="chapter4.html">
>    <link rel="next" href="chapter6.html">
>  </head>
> Ultimately for the @about="" to play the role of a guard it needs to
> be placed as 'close' as possible to the entries you are trying to
> protect (the <link> and <meta> elements inside <head>) and that means
> having the implied @about="" on <head>.
> The same logic goes for <body>, by the way. This is because 'vanilla'
> (pre-RDFa) HTML also allowed metadata to be added to the body of a
> document; the metadata we expressed above could also be marked up in
> the following way:
>    <h1>Chapter 5</h1>
>    <a rel="prev" href="chapter4.html">Prev</a>
>    |
>    <a rel="next" href="chapter6.html">Next</a>
> Once again, adding @typeof to <body> would throw all of this markup
> out, causing the predicates to become attached to the 'new' bnode
> instead of the 'current document'.
> I don't think there is any doubt that a solution to this problem was
> needed, since otherwise the triples about a document would have simply
> 'disappeared' the moment you indicated what type a document was. But
> whether the solution we have is the right one is difficult to tell.
> One minor variation of the technique we decided on would have been to
> have said that <head> and <body> actually set the subject to the
> current document in the processing and that's that, i.e., without
> using an implied @about. However, I think it was generally felt that
> having an 'implied @about' was a better solution since it gave authors
> the ability to override the value if they wanted to, by providing an
> explicit @about.
> A completely different approach would have been to simply say that the
> <html>, <head> and <body> elements are not allowed to take RDFa
> attributes, and if they do then those attributes are ignored. However,
> I think at the time we felt there might be other scenarios where you
> would want to put attributes on those three elements, so having the
> implied @about seemed to be the smallest change we could make to the
> processing rules whilst still solving the problem.
> I did a quick search in the old mailing-lists to find something that
> might capture the essence of our discussion back then, just in case
> anybody wanted to re-read! I think [2] is the best post since it links
> to a lot of the prior discussions that show something of what we were
> trying to grapple with.
> Regards,
> Mark
> [1] <http://www.w3.org/TR/html4/struct/links.html#h-12.1.2>
> [2] <http://lists.w3.org/Archives/Public/public-rdf-in-xhtml-tf/2007Oct/0252.html>

Ivan Herman, W3C Semantic Web Activity Lead
Home: http://www.w3.org/People/Ivan/
mobile: +31-641044153
FOAF: http://www.ivan-herman.net/foaf.rdf
Received on Sunday, 20 November 2011 12:14:28 UTC

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