value-class-pattern: Difference between revisions

Revision as of 19:29, 1 August 2009

<entry-title>Value Class Pattern</entry-title>

The value class pattern is derived from value-excerpting in hCard.

The editors believe the value-class-pattern to be feature complete and ready for use in markup. Implementers are encouraged to update accordingly and provide feedback. Note, the precise parsing behavior could change in response to implementer feedback, but the core methods are stable. You should watch this page for updates.

Simple Examples

Here is markup for a home phone number:

vCard fragment:

TEL;TYPE=HOME:+1.415.555.1212

hCard fragment:

 <span class="tel">
   <span class="type">Home</span>:
   <span class="value">+1.415.555.1212</span>
 </span>

In this case, the value of tel is +1.415.555.1212, not Home: +1.415.555.1212.

Sometimes the value for a microformats property must be split into multiple pieces in the content of the element representing that property. Multiple elements with a class name of "value" (value elements) can be used to extract and concatenate these pieces into a single value for microformats properties which expect simple strings or tel values.

Another example, this time using a localized (British) telephone number:

 <span class="tel">
   <span class="type">Home</span>:
   <span class="value">+44</span> (0) <span class="value">1223 123 123</span>
 </span>

In this case, the valid data for the telephone number is +441223123123, but the way in which phone number is presented in Britain will include the (0), for local dialling. That is, from anywhere in the world you may dial +441223123123, or from within Britain you may dial 01223123123. Common local publishing interferes with the data, since dialling +4401223123123 is an invalid number.

In the mark-up, two value classes target the part of the telephone number string that makes an international, valid number, whilst allowing conventional presentation.

Another example, adding a place name to a geo co-ordinate:

<p>I'm loitering outside The Bricklayer's Arms
  <span class="geo">
    51° 30' 48.45", -0° 8' 53.23"
    (<span class="value">51.513458;-0.14812</span>)
  </span>
</p>

Whilst the entire string is a geo point, it's only the decimal encoded co-ordinates which must be consumed by a microformats parser, so the value class isolates it from the degrees form, which the publisher includes for completeness.

Basic Parsing

The value class pattern only applies to properties which are simple strings, enumerated values, telephone numbers, and datetimes. The value class pattern does not affect parsing of properties of type email, URL, URI, UID.
Where an element with such a microformat property class name has a descendant with class name value (a "value" element), parsers should use the following portion of that element:
1. if the value element is an img or area element, then use the element's alt attribute value.
2. if the value element is an abbr element, then use the element's title attribute value.
3. for any other element, use its inner-text.
Where there are multiple descendants of a property with class name of value (multiple value elements)
1. if the microformats property expects a simple string, enumerated value, or telephone number, then the values extracted from the value elements should be concatenated without inserting additional characters or white-space.
2. if the microformats property expects a datetime value, see the Date Time Parsing section.
Descendants with class of value must not be parsed deeper than one level. That is, where an element foo with class value has a descendant bar with class value, the content of foo is taken as the value. Nesting additional elements with class of value cannot be used to further isolate a property's value.

e.g.

 <p class="description">
  <span class="value">
    <em class="value">Puppies Rule!</em>
    <strong>But kittens are better!</strong>
 </span>
</p>

In this example, description has a child ‘value’, and that child has a grandchild ‘value’. However, the parsing of value classes stops at the first level, so the data for description is: Puppies Rule!But kittens are better!, not just Puppies Rule!.

Date and time values

Some microformats properties expect an ISO8601 datetime value, e.g. hCalendar dtstart and dtend or hAtom published.

Authors may use the value class pattern to separately specify the date and the time, which are then combined to specify a single datetime value.

Example, this hCalendar 'dtstart' property with 'value' elements:

<p>The weekly dinner will be on 
    <span class="dtstart">
        <abbr class="value" title="2008-06-24">this Tuesday</abbr> 
     at <span class="value">18:30</span>
    </span>
</p>

produces the following 'dtstart' value:

2008-06-24T18:30:00

and iCalendar converters produce the following DTSTART:

DTSTART:20080624T183000

The lack of a timezone indicates a "floating" datetime, that is a datetime independent of any particular timezone. Examples of floating datetimes could be an alarm clock you set to ring at 7am, or the common 9am-5pm workday.

Date and time parsing

For all date time properties (as defined in their respective microformats specifications), the following rules apply in addition to (and in some cases replacing) the above value class pattern parsing rules.

When a "value" element is found, parse a value from the element as follows:

if the element is an img or area element, then use the element's alt attribute value.
if the element is an abbr element, then use the element's title attribute value.
for any other element, use its inner-text.
if the value has both a specific ISO8601 date and a specific time, use those and stop looking for "value" elements.
if the value has *only* a specific date, specifically, fits the following ISO8601 date patterns (i.e. as documented in the Wikipedia summary of ISO8601)
- YYYY-MM-DD
- YYYY-DDD
- then use that as the date value. For the purposes of the value class pattern, the hyphens "-" separating the year, month, day and/or ordinal day are required.
- ignore any further "value" elements that specify the date.
if the value has *only* a specific time (with or without timezone), parse it for a time value that can match any of the following:
- HH:MM:SS-XX:YY
- HH:MM:SS+XX:YY
- HH:MM:SS-XXYY
- HH:MM:SS+XXYY
- HH:MM:SSZ
- HH:MM:SS
- HH:MM-XX:YY
- HH:MM+XX:YY
- HH:MM-XXYY
- HH:MM+XXYY
- HH:MMZ
- HH:MM
- HH is the 24 hour "hours" in the time, from 00 to 24, with optional leading 0 for values less than 10.
- MM are the minutes from 00 to 59
- SS are the optional seconds from 00 to 59 (60 for a leap second). If omitted, infer 00.
- XX is the time zone hours offset, from 00 to 12 with optional leading 0 for values less than 10.
- YY is the time zone minutes offset, from 00 to 59, though in practice only 00, 15, 30, 45 minute offsets are used in global timezones.
- Z is the literal 'Z' to indicate GMT.
- For the purposes of the value class pattern, the colons ":" separating the hour, minutes, seconds are required.
- However the colons ":" separating the hours and minutes of any timezone offset are optional and discouraged in order to make it less likely that a timezone offset will be confused for a time.
- (NOTE: consider case insensitive { }"am"|{ }"a.m." suffix to treat an HH value of 12 as 00, or a case-insensitive { }"pm"|{ }"p.m." suffix to add 12 to HH value less than 12 - per Wikipedia article on the 12 hour clock)
- ignore any further "value" elements that specify the time.
if the value has *only* a specific timezone, parse it for a time zone value that can match any of the following:
- -XX:YY
- +XX:YY
- -XXYY
- +XXYY
- -XX
- +XX
- Z
- XX is the time zone hours offset, from 00 to 12 with optional leading 0 for values less than 10.
- YY is the time zone minutes offset, from 00 to 59, though in practice only 00, 15, 30, 45 minute offsets are used in global timezones.
- Z is the literal 'Z' to indicate GMT.
- ignore any further "value" elements that specify the timezone.

If by parsing the "value" element(s), at least a specific date has been found, then the "value" is overall valid, and the parser assembles the overall datetime value by concatenating the specific date, "T" and specific time (if time was specified, with 00 seconds implied if no seconds are provided), and specific timezone (if timezone and a specific time was specified).

YYYY-MM-DD - no time specified
YYYY-MM-DDTHH:MM:SS - time specified but no timezone. This is a floating time.
YYYY-MM-DDTHH:MM:SS-XXYY or
YYYY-MM-DDTHH:MM:SSZ or
YYYY-MM-DDTHH:MM:SS+XXYY - both time and timezone were specified.

iCalendar converters in addition must do the following:

remove any dash "-" separators in the date.
remove any colon ":" separators in the time.
perform datetime math on any +/- relative timezone value, and produce an effective UTC value ending with "Z".

Atom converters in addition must do the following:

normalize all date and datetime values to RFC 3339.

derivation and tests

The handling of date and time values in the value class pattern was originally brainstormed on the value-excerption-pattern-brainstorming page and derived from that analysis and feedback. For the curious, historical details may be found there, along with additional thoughts for extension.

See value-class-date-time-tests for test cases.

format specific optimizations

The following are format specific optimizations under consideration that make use of the value-class-pattern.

short URL for this section: http://tr.im/vcpfso

hCalendar dtend implied date

Typically events that start and end the same day only display the date of the event once (makes sense per the DRY principle) (real world examples: Upcoming, ... more examples with URLs would help for thoroughness).

Thus it would be convenient if we could imply an hCalendar event "dtend" date from its "dtstart" date when only the time (and optionally timezone) was specified for its "dtend", e.g.:

<span class="vevent">
 The <span class="summary">party</span> will be on 
 <span class="dtstart">
  <span class="value">2009-06-26</span>, from
  <span class="value">19:00</span></span> to 
 <span class="dtend"><span class="value">22:00</span></span>.
</span>

To simplify this further for authors, hCalendar processors could treat the specifying of just the time per the value-class-pattern date and time value rules, and thus eliminate the need for the "value" span inside the "dtend" span:

<span class="vevent">
 The <span class="summary">party</span> will be on 
 <span class="dtstart">
  <span class="value">2009-06-26</span>, from
  <span class="value">19:00</span></span> to 
 <span class="dtend">22:00</span>.
</span>

hCalendar to iCalendar converters should produce the following iCalendar fragment (as part of a valid .ics file) from either of the above two examples:

BEGIN:VEVENT
SUMMARY:party
DTSTART:20090626T190000
DTEND:20090626T220000
END:VEVENT

hAtom updated implied date

Similarly, in blog posts that indicate both when they were "published" and "updated", the date is usually only displayed once, typically when "published" (real world examples with URLs would help for thoroughness).

Thus it would be convenient if we could imply an hAtom entry "updated" date from its "published" date when only the time (and optionally timezone) was specified for its "updated", e.g. for a blog post that was updated the same day:

<span class="hentry">
 <span class="entry-summary">short blog post example</span>
 was published on <span class="published">
  <span class="value">2009-08-01</span> at <span class="value">12:06</span></span>
 and updated at <span class="updated"><span class="value">12:10</span></span>.
</span>

To simplify this further for authors, hAtom processors could treat the specifying of just the time per the value-class-pattern date and time value rules, and thus eliminate the need for the "value" span inside the "updated" span:

<span class="hentry">
 <span class="entry-summary">short blog post example</span>
 was published on <span class="published">
  <span class="value">2009-08-01</span> at <span class="value">12:06</span></span>
 and updated at <span class="updated">12:10</span>.
</span>

Parsing value from a `title` attribute

The value-title class name allows the publisher to indicate the data value for a parent property is contained in the title attribute of an element, rather than the inner-text.

This can be used to provide a synonym within content, or used to quietly publish alternate forms of information for microformats parsing, without affecting the consumption of content.

For example, you can use casual localization with dates:

<p>It was 
 <span class='dtstart'>
  <span class='value-title' title='2008'>last year</span>
 </span>
  that I realised my addiction to cashew nuts would cost this country so dear.
</p>

Parsing rules for value-title are the same as for value above, with the following change:

Where a microformats property has a child element with class name of value-title, the content of the title attribute of that element must be parsed, rather than the portion of the element that would be parsed for a class name of value.

Using `value-title` to publish machine-data

The initial usage of value-title is used to publish alternate, parsable forms of property values in a visible context without the use of the abbr element whose semantics already support interpretation of the 'title' attribute as an expanded, more precise form of the content.

Experience has found that there are some cases in microformats where a number of publishers want to include a precisely accurate and parsable value for a property but do not want it to be visible in their page, even as a tooltip.

For example, full ISO8601 datetimes may be confusing to readers of the page (as a tooltip or when read aloud by a screen reader), and enumerated values such as the type subproperty of hCard's tel property use US-English terms, which are not part of pages in any other language.

Since both of those scenarios have shown to be obstacles for a number of publishers, for these cases, and these alone, there exists a further extension of value-excerption. This extension allows the parsable form of the property to be published ‘silently’ immediately adjacent with the respective local visible content.

Here is an example, with the required use of a first child element with class name value-title:

<p class='tel' lang='en-gb'>
  <span class='type'>
    <span class='value-title' title='cell'> </span>
      mobile
    </span>
  <span class='value'>+44 7773 000 000</span>
</p>

The cell value is parsed for the 'type' subproperty, but mobile is presented to the user.

In the case of dates:

<p class='dtstart'>
  <span class='value-title' title='2009-03-14T16:28-0600'> </span>
  March 14th 2009, around half-past four
</p>

A microformats parser will read the ISO8601 format datetime 2009-03-14T16:28-0600, but users will only see March 14th 2009, around half-past four. Testing has shown that the ISO8601 datetime above does not get exposed to any user at all.

Parsing machine-data `value-title`

Browsers collapse the value-title span down to a width of 0, effectively providing no visual rendering, whilst keeping the element in the DOM. With no physical dimensions, there is no ‘hover’ state, so no tooltip is revealed. Furthermore, the empty element is not passed to assistive technology layers such as VoiceOver. Screen readers do not read the contents of the title attribute of an empty span element.

We conducted thorough testing of these parsing behaviors to ensure accessibility.

Note: Whilst the value-title element is more gracefully written without whitespace inner-text (or as self-closing <foo /> element in XHTML), current tools such as WYSIWYG editors and HTML-Tidy will erroneously discard such elements, resulting in parsable data being thrown away by some tools. As such,  , including a single whitespace character between the opening and closing tag, is the required pattern for authors, at this time.

Parsing this final value-title extension imposes some stricter restrictions on usage. These restrictions exist to reduce the impact of DRY violations, reduce the opportunity for sites to spoof data, and encourage best practice for maintaining both forms of data accurately.

Where an element with class value-title is to be parsed as data for a property, and that element also contains no non-whitespace content (hereafter referred to as ‘empty’), the following rules apply:

The ‘empty’ value-title element must be the first, non-whitespace child of the property element. That is, it should follow immediately after the property is declared, before the human-readable form, and without any additional nesting.
The ‘empty’ value-title element can only be used for specific properties. Microformat specifications must explicitly state which properties may be used with this extension of the value-class-pattern.
Where an ‘empty’ value-title element is to be used as the single property value, it must be the only such value content. That is, the first instance of a conforming value-title element overrides all other value and value-title siblings and/or cousins.
Tools written to perform Conformance Testing and/or Validation of microformats should attempt to compare the machine-data and human legible forms of the property data, and advise authors if the forms do not match.

limited use of value-title

Due to the fact that the value-title pattern hides some amount of data which tends to be a machine-specific duplicate of data that is provided in the human readable content, there are two microformats principles being compromised: visibility and DRY. Thus the applicability of this pattern is deliberately restricted to properties that have demonstrated through experience a need for it, with no known better alternative.

In general authors should:

First, try to directly specify microformats property values inline (the most visible, no duplication),
Then consider using the value-class pattern
1. Including multiple value elements for date and time properties
and then only if those methods are insufficient, consider the value-title pattern.

This document post-dates other microformat specifications, such that they may not yet indicate which properties permit use of this pattern. In the interim, only the following types of properties should allow the value-title pattern.

ISO8601 date, datetime, timezone, and duration values
Enumerated values (such as the hCard tel/email/adr 'type' subproperties)
Co-ordinates (such as the geo 'latitude' and 'longitude' properties)
Telephone number properties (e.g. the hCard 'tel' property)

The machine-data page has documentation of some of the properties of some specs which experience has shown need a solution like the value-title pattern.

There are some simple reference examples and tests for this pattern on value-class-pattern-tests.

In future use, specification authors may inherit use of value-title by use of ISO8601 date and time formats, or reuse of other microformats, but specifications should _avoid_ introducing new data structures that depend on or encourage this pattern. New specifications are themselves expected to adhere to the principals of visibile data and DRY.