machine-data

Machine Data in Microformats

Microformats are designed to mark-up human consumable information, as commonly found in the wild. But, in a number of exceptional cases it has been necessary to specify precise data formats for particular properties. Formats for dates, times and locations are standardised in a way that doesn't always match the way information is visibly published. This is necessary to make the data understandable to parsers. Similarly, there are keywords in hCard that must be written in English (telephone ‘type’ in hCard, for example).

It is necessary for these data formats to be fixed to make the data parsable by machines; the cost for a parser to support every commonly published date-time format in the world (include approximations like ‘five minutes ago’) is too high, as is handling international translation (such as mobile telephones; US-English ‘cell’ published as British English ‘mobile’).

In some cases, the human version of the data can be semantically described as an abbreviated form of the machine data, and the machine data may also be human consumable. For example, the date-design-pattern uses HTML's abbr element to expand one human date representation into the ISO 8601 form date: ‘January 1st’ is an abbreviated form of ‘2008-01-01’. The latter is also legible to humans (and can be exposed to them through tool-tips and assistive screen readers).

In other cases, this machine data is not legible to humans. In hAudio, the duration property uses ISO 8601, resulting in machine data of PT3M23S; not understandable to humans, and therefore not a valid expansion of ‘three minutes and twenty-three seconds’.

Cases of Fixed Data Formats in Microformats

The following are all current uses of fixed format machine data required by the various microformats.

hCalendar

• Uses ISO 8601 for dtstart, dtend, duration and rdate

hCard

• Telephone type keywords: voice, home, msg, work, pref, fax, cell, video, pager, bbs, modem, car, isdn, pcs.
• Address type keywords: INTL, POSTAL, PARCEL, WORK, dom, home, pref.
• Email type keywords: INTERNET, x400, pref.
• Uses ISO 8601 date for bday
• ISO 8601 time zone for tz
• Telephone numbers requires a numerical form, whilst phone numbers can be presented in alpha-numeric form: e.g. +1-555-FORMATS

hReview

• Uses an ISO 8601 date-time for dtreviewed
• Uses fixed-point integer values from 0-5 for rating (publishers may, for example, display a percentage rating)

hAtom

• Uses ISO 8601 date-time for updated
• Uses ISO 8601 date-time for published

hResume

• Uses ISO 8601 date for individual experience items.
• Uses ISO 8601 date for individual education items.

Geo

• Requires latitude and longitude in decimal form (1.23232;-2.343535), but may be published in degrees: N 37° 24.491, W 122° 08.313
• Locations are most often published just as place names (not abbreviated co-ordinates)

hAudio

• Uses ISO 8601 for track duration, e.g. PT3M23S

Embedding Fixed Data Formats in Microformats

There are currently three supported methods of including these fixed data formats in a microformatted document.

As Visible Page Content

You may use the standard class-design-pattern to mark-up the data visibly in the page.

Ben was born on <span class="bday">1984-02-09</span>.

We're meeting up on Northumberland Avenue (<span class="geo">51.507033,-0.126343</span>).

As An Abbreviation

In some cases, the data formats specified make valid expansions of common human forms, such as dates in in an hCard birthday field:

Ben was born on <abbr class="bday" title="1984-02-09">9th February</abbr>

Note, however, that not all data formats are valid expansions. In HTML, the abbr element is working semantically at a text level, not a data level. Both the abbreviated form (the inner text) and the expanded form (the title) need to be consumable by humans.

This means that in hAudio, using an abbreviation for duration is incorrect:

<abbr class="duration" title="PT3M23S">3 minutes, 23 seconds</abbr>

Whilst the data ‘PT3M23S’ is an expanded form of ‘3 minutes, 23 seconds’, the text is not; ‘PT3M23S’ is nonsense to most human beings. abbr is an element that describes the text, not the data. HTML4 has no way to mark up arbitrary data.

As Supplementary Data using the value-excerption-pattern

The machine data form can be included alongside any human legible text, and hidden using another layer of the browser stack (namely, CSS). This behaviour is documented as the value-excerption-pattern, and derived from the value excerpting behaviour in hCard.

So, for example, when describing a location by name, but still wanting to include geo for the machine-readable location:

<span class="geo">Northumberland Avenue, London <span class="value">51.507033,-0.126343</span></span>

Then, optionally use CSS to hide the data you don't want displayed:

.geo > .value { display: none; }

The same pattern works for the hAudio duration example given above:

<span class="duration">3 minutes and 23 seconds <span class="value">PT3M23S</span></span>

And again, the optional CSS:

.haudio .duration > .value { display: none; }

Of course, this does result in a dependency on CSS to make the data invisible to users, and will result in the machine data being displayed alongside the human form in any user agent without CSS support. That's a compromise that has to be resolved based on the requirements of individual sites.

Proposed Methods

As Invisible Supplementary Data

This section is a proposed extension to value-excerpting, is currently open to active discussion and is not currently supported in parsers. You must not implement this in pages at this time.

Value excerpting is already implemented as a means of extracting data from within a microformat property. Where the element (any element) with class="value" is also empty (containing no inner-text), parsers should instead read the value of the title attribute of that element.

So, the following code will read the inner-text of the element, as per current implementations:

<span class="dtstart">Tomorrow lunchtime <span class="value">2008-05-17T12:00:00+0100</span></span>

The data format, poorly legible to most humans remains visible in the page. To make the machine data invisible at an HTML level, the following can also be parsed:

<span class="dtstart">Tomorrow lunchtime <span class="value" title="2008-05-17T12:00:00+0100"></span></span>

The span with class="value" is empty, therefore the parser must read the value of the title attribute instead. Where the element is not empty, the title attribute is ignored.

Empty elements are invisible within the page, and take up no physical space, so the title is not exposed as a tool-tip. Also, the entire element is ignored by assistive technology such as screen readers, therefore the data is not exposed to any user. This assistive technology claim is based on informed, expert advice, but is awaiting confirmation through testing. That testing is forthcoming.

HTML has no pure way of including machine data inline. The use of value excerpting, which can be applied to any HTML element, is the least obtrusive way to embed data into HTML, without overloading existing element semantics and without browser compatibility issues.

Potential Problems

• Some parsers (particularly those that run incoming HTML through Tidy to convert it into well-formed XML) may strip empty inline elements. A workaround may be to allow (or even require) hard white space (i.e.  ) within the element with class='value".
  • It is, however, trivial to patch and build Tidy not to do this (keeping empty elements where that element also has a class attribute). Parser writers need to feed back on whether using a custom build is impossible to their solution, but since Tidy can be made to work, the problem can likely be alleviated. Ben Ward has put up an experimental build of Tidy with patched element-dropping behaviour here: tidy-microformats.zip

Other Proposals

• Use a ‘ufusetitle’ class name to particular elements, as a processing instruction to parsers to read the title attribute rather than inner text. However, this adds a new concept of putting ‘parsing instructions’ into the class attribute, which is currently only used to extend the semantics of elements.
• Break with requirements for valid HTML and adopt the RDFa content attribute as a means of embedding data (or another custom attribute). This results in invalid HTML.
• Continue using abbr for all data embedding, but prefix the data with the string data:, so that users exposed to the data string are given some context. This reduces the impact of the problem at the user side, but does not solve the problem of the machine data being exposed to human users in the first place.
• Repurpose the input element with a type=hidden attribute. Browsers hide this element from users completely. However, this would stretch the semantics of input; using an input, forms device for output.

Acknowledgements

• James Craig and Bruce Lawson for the suggestion using an empty span as a means of embedding data without it being exposed in assistive technology
• Jeremy Keith for assistance refining the invisible data extension to value-excerpting

Related Pages

• value-excerption-pattern
• class-design-pattern
• abbr-design-pattern
• date-design-pattern
• datetime-design-pattern
• HTML 4.01 definition of <abbr> element