[whatwg] Trying to work out the problems solved by RDFa

Toby A Inkster mail at tobyinkster.co.uk
Thu Jan 1 07:24:50 PST 2009

The use cases for RDFa are pretty much the same as those for  

For example, if a person's name and contact details are marked up on  
a web page using hCard, the user-agent can offer to, say, add the  
person to your address book, or add them as a friend on a social  
networking site, or add a reminder about that person's birthday to  
your calendar.

If an event is marked up on a web page using hCalendar, then the user- 
agent could offer to add it to a calendar, or provide the user with a  
map of its location, or add it to a timeline that the user is  
building for their school history project.

Providing rich semantics for the information on a web page allows the  
user-agent to know what's on a page, and step in and perform helpful  
tasks for the user.

So why RDFa and not Microformats?

Firstly, RDFa provides a single unified parsing algorithm that  
Microformats do not. Separate parsers need to be created for  
hCalendar, hReview, hCard, etc, as each Microformat has its own  
unique parsing quirks. For example, hCard has N-optimisation and ORG- 
optimisation which aren't found in hCalendar. With RDFa, a single  
algorithm is used to parse everything: contacts, events, places,  
cars, songs, whatever.

Secondly, as the result of having one single parsing algorithm,  
decentralised development is possible. If I want a way of marking up  
my iguana collection semantically, I can develop that vocabulary  
without having to go through a central authority. Because URIs are  
used to identify vocabulary terms, I can be sure that my vocabulary  
won't clash with other people's vocabularies. It can be argued that  
going through a community to develop vocabularies is beneficial, as  
it allows the vocabulary to be built by "many minds" - RDFa does not  
prevent this, it just gives people alternatives to community  

Lastly, there are a lot of parsing ambiguities for many Microformats.  
One area which is especially fraught is that of scoping. The editors  
of many current draft Microformats[1] would like to allow page  
authors to embed licensing data - e.g. to say that a particular  
recipe for a pie is licensed under a Creative Commons licence.  
However, it has been noted that the current rel=license Microformat  
can not be re-used within these drafts, because virtually all  
existing rel=license implementations will just assume that the  
license applies to the whole page rather than just part of it. RDFa  
has strong and unambiguous rules for scoping - a license, for  
example, could apply to a section of the page, or one particular image.

RDFa was largely borne of looking at Microformats, looking at what  
was successful about them, considering problems with them, and  
finding ways to resolve those problems.

1. It has been discussed in hAudio, figure, hRecipe and others.

Toby A Inkster
<mailto:mail at tobyinkster.co.uk>

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