[whatwg] namespaces in html5

Ian Hickson ian at hixie.ch
Fri Jan 20 11:24:42 PST 2012

On Mon, 18 Jul 2011, David Karger wrote:
> [...] the Exhibit data visualization framework 
> (http://simile-widgets.org/exhibit) [...]
> The goal of Exhibit is to make it easy for non-programmers to embed 
> interactive data visualizations in their web pages.

HTML has a number of features intended for such things. The class 
attribute, for example, could be used to flag a table as something that 
should get a graph:

   <table class="graph-me">...</table>

Specific annotations for the graphing script can be included in data-*="" 
attributes; for example, this:

   <table class="graph-me" data-graphs-type="xy">
     <th data-graphs-series-kind="x time-series">Date</th>
     <th data-graphs-series-kind="y log">Date</th>

...might be how you mark up the top of a table that's going to be drawn as 
an X-Y plot with a time-based x axis and a logarithmic y axis.

You can use <meta> to include page-wide information. You can link to other 
resources using <link rel> or <a rel>. You can embed raw data using 
<script type>, for example, assuming the type was registered:

   <script type="text/graph-data">
    { type: 'xy', x: 'time-series', y: 'log',
      data: [...] }

If the data structure is more like nested name-value lists than tabular, 
you could use microdata to mark it up, with the script then using the 
microdata DOM API to present the data.

In short, there are a huge number of ways to approach this.

We are also working on further options. The component work in the Web Apps 
working group is developing mechanisms for encapsulating widget 
definitions, so that your script could bind directly to the data in the 
page. This same work will likely involve introducing author-extensible CSS 
properties for styling purposes, as well.

> Another approach would be to use the catchall html5 data- prefix for 
> attributes.  We could certainly prefix all of our specialized attributes 
> with the data- prefix, which would turn those attributes valid for html.  
> This solution is unsatisfactory for two reasons.  The first is that our 
> attributes are not data attributes----we are not using 
> microformat-oriented data attributes; rather, we are using attributes 
> that describe visualizations. data- seems a poor choice of prefix.

Treat the five characters "data-" as an opaque string. data-* attributes 
are for use by scripts for any purposes that the script wants.

Personally I would recommend against putting presentational information in 
the markup -- whether you use XML namespaces, data-* attributes, or 
non-conforming attributes of your own invention. The right place for 
styling information is CSS. On the long run, as mentioned above, I expect 
we'll provide explicit hooks in CSS for authors to put custom style 
information for this purpose (the equivalent of data-* attributes but for 
properties). Unfortunately we're not there yet.

> The second problem that concerns me is attribute collisions.  If we use 
> an attribute like data-role="view", how long will it be before an 
> exhibit author runs into a situation where a different javascript 
> library is using the same data-role attribute for a different purpose, 
> which would make the two libraries incompatible with one another?

Just use the format data-exhibit-foo="".

> I have no specific loyalty to namespaces, but I am really hopeful that 
> html5 will offer us a solution that reflects the issues I outlined 
> above, namely:
> * allow extension of them html5 vocabulary with attributes Exhibit will 
> use to anchor visualizations,
> * such that the resulting html will validate,
> * without requiring rigid obedience to the challenging html polyglot 
> syntax, which is beyond the capabilities of our target novice web 
> authors
> * and protecting us from a future in which collisions on choice of 
> attribute names make our library/vocabulary incompatible with others'

That's what data-library-name="value" attributes are for. They exactly fit 
the described requirements.

On Mon, 18 Jul 2011, Anne van Kesteren wrote:
> You could use data-exhibit-* as the specification suggests. Potentially 
> including the ability for the web author to override the "exhibit" 
> constant.

Indeed. That's exactly what data-*="" is for: passing data to a script 

On Mon, 18 Jul 2011, David Karger wrote:
> Yes, we could,  but it doesn't address the two objections I raised to data-
> prefix:
> 1.  it isn't actually a data attribute, so prefixing with data seems odd
> (appearance; minor)

This is a non-issue. The attributes could be called carrot-*="" or 
socialism-*="" or presentation-*="", what matters is what their definition 
says, not what they are called. (Most users of HTML don't speak English as 
their first language...)

> 2.  there's no way to guarantee someone else won't use the same 
> data-exhibit prefix, causing incompatibilities (functionality; major)

This is, in practice, a trivial problem. It turns out that there are 
relatively few libraries, and so the odds of two libraries picking the 
same short string library name and then being used by the same person is 
very low. (It's about the same risk as two people picking the same library 
name. Does the lack of namespacing for library names cause you a problem?)

On Mon, 18 Jul 2011, Tab Atkins Jr. wrote:
> You seem to have mentally associated the data-* attributes with 
> Microdata.  There is no connection between them.  In fact, it's 
> impossible for Microdata to use the data-* attributes at all.


> data-* attributes are for private script data that is, for whatever 
> reason, more convenient to attach directly to a DOM node than to hold in 
> a JS structure.  Wanting the data's link to DOM nodes to survive 
> serialization is a good reason.


> > 2.  there's no way to guarantee someone else won't use the same 
> > data-exhibit prefix, causing incompatibilities (functionality; major)
> In practice, the risk of prefix collisions has turned out to be minimal 
> in many real-world collections, such as jQuery plugins.  We expect the 
> same to apply here.  For maximum robustness, simply write your library 
> with the ability to accept a different prefix, so that if a collision 
> does occur the author can work around it.


On Mon, 18 Jul 2011, David Karger wrote:
> The html5 spec states that
> > 
> > Custom data attributes 
> > <http://dev.w3.org/html5/spec/elements.html#custom-data-attribute> are 
> > intended to store custom data private to the page or application, for 
> > which there are no more appropriate attributes or elements.
> > 
> > These attributes are not intended for use by software that is 
> > independent of the site that uses the attributes.
> > 
> and further
> > 
> > It would be inappropriate, however, for the user to use _generic 
> > software not associated with_ that music site to search for tracks of 
> > a certain length by looking at this data.
> > 
> > This is because these attributes are intended for use by the site's 
> > own scripts, and are not a generic extension mechanism for 
> > publicly-usable metadata.
> As I interpret these words, data- attributes are intended to be 
> delivered by a server for use by the javascript code that server 
> delivers with the page.


> The exhibit attributes are not associated with any server, and are not 
> associated with any particular data items being delivered by any server. 
> Rather, they are part of "generic software not associated with" the 
> server (see quote above)  and handle _presentation_ of the content on 
> the page.
> So, while it might be technically valid to use data- prefixes, it 
> doesn't seem to fit the intention.

Is the exhibit JavaScript library not being delivered with the page?

Ian Hickson               U+1047E                )\._.,--....,'``.    fL
http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'

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