W3C home > Mailing lists > Public > public-vocabs@w3.org > June 2012

additionalType property, vs extending Microdata syntax for multiple types

From: Dan Brickley <danbri@danbri.org>
Date: Fri, 15 Jun 2012 20:26:03 +0200
Message-ID: <CAFfrAFpbhTkGoY8_PVK-d+4EvkD=bBxXNVRubxJG8wR7MSWB5Q@mail.gmail.com>
To: public-vocabs@w3.org
Cc: "Martin Hepp (UniBW)" <martin.hepp@ebusiness-unibw.org>, Peter Mika <pmika@yahoo-inc.com>, Ramanathan Guha <guha@google.com>, Jeni Tennison <jeni@jenitennison.com>
HTML5 Microdata, as defined in

... has only limited support for describing multiple types that
something belongs to. In particular it requires they are described
using a single schema.

For Good Relations integration (and other scenarios) people have asked
for a way of listing more types within schema.org markup.

* One model is to use RDFa 1.1 (Lite), where this is quite natural.
* Another is to add (as a workaround) a new property, e.g. called
'type' or 'additionalType', to schema.org's vocab (Martin requests
this in http://www.w3.org/wiki/WebSchemas/GoodRelations )
* A 3rd is to stretch
to allow different namespaces to be used.

The note at http://www.w3.org/TR/html-data-guide/#multiple-types-microdata
discusses just this issue.

I'm sending this as followup from discussion amongst schema.org
partners, who welcome community advise on this point. Should we add a
'type' property to schema.org, try to change or stretch microdata
syntax, ... or encourage people who want multiple diverse types to use
RDFa Lite instead?


Copied below is the current Microdata / HTML5 spec text,

"The item types of an item are the tokens obtained by splitting the
element's itemtype attribute's value on spaces. If the itemtype
attribute is missing or parsing it in this way finds no tokens, the
item is said to have no item types.

The item types must all be types defined in applicable specifications
and must all be defined to use the same vocabulary.

Except if otherwise specified by that specification, the URLs given as
the item types should not be automatically dereferenced.

A specification could define that its item type can be derefenced to
provide the user with help information, for example. In fact,
vocabulary authors are encouraged to provide useful information at the
given URL.

Item types are opaque identifiers, and user agents must not
dereference unknown item types, or otherwise deconstruct them, in
order to determine how to process items that use them.

The itemtype attribute must not be specified on elements that do not
have an itemscope attribute specified."
Received on Friday, 15 June 2012 18:26:32 UTC

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