W3C home > Mailing lists > Public > public-html@w3.org > January 2010

Issue 83 Change Proposal - caption attribute

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Wed, 13 Jan 2010 14:57:16 -0600
Message-ID: <dd0fbad1001131257o36f151e8j95512a5ed88f4d35@mail.gmail.com>
To: public-html <public-html@w3.org>
I have created a change proposal for one suggested resolution to issue
83 in w3 space, located at
http://esw.w3.org/topic/HTML/ChangeProposals/DdDtCaptionAttribute.
For convenience, I reproduce the current text below:


Summary
=======

The proposal is to introduce a caption attribute that may be used on
an element within figure to denote the contents of that element as
being the caption. This proposal does not address the details element.


Rationale
=========

1. This allows the most intuitive name to be used, without the
problems caused by actually using <caption> as an element name.

2. It roughly parallels the existing uses of attributes to tie
together an element and some containing element with additional
semantics, as seen in <time pubdate>.

3. It does not alter the existing semantics of the element it is
applied to; a <ul caption> is still a <ul>, a <time caption> is still
a <time>, etc.

4. It forgoes the need for a 'wrapper' element when you just want to
immediately apply a special element. For example, if you are putting
photographs in a figure, with the only caption being the time it was
taken, it would be best if you could simply put in the <time> without
a wrapper. That is, <figure><img src=foo><time caption
datetime=2010-01-11>January 11th, 2010</time></figure> is preferable
to <figure><img src=foo><caption><time datetime=2010-01-11>January
11th, 2010</time></caption></figure>


Problems with Using dt/dd for figure and details Elements
=========================================================

These considerations are also relevant for many other proposals for
resolving this issue.

Backwards Compatibility
-----------------------

Due to IE's legacy parsing issues, the dt and dd elements create
problems for authors attempting to use and style these elements. The
simplest workaround (wrapping both figure and details in an extra div)
that authors would need to apply to avoid these issues is not
intuitive, as clearly demonstrated by the wide range of alternative
solutions that were attempted prior to its discovery.

Legacy Cruft
------------

The use of extra div elements may lead to a lot of confusion among
authors, at least initially, and ultimately end up becoming extraneous
markup that authors just include without really understanding why,
even after the current generation of browsers are long since dead.

We've seen this kind of problem before with authors, for example,
frequently wrapping scripts with pseudo-HTML-comments initially
intended to hide the script from what are now very ancient browsers,
none of which have been in use for over a decade.

Styling Clashes
---------------

In addition to the aforementioned styling bug in IE, many existing
stylesheets already include styles for the dt and dd elements, used in
description lists (dl elements). Authors attempting to incorporate
figure or details into their existing pages would have to take care to
avoid unintended styling clashes caused by selectors that aren't
specific enough. i.e. It's common for authors to simply use the
selector as "dt" or "dd", rather than "dl>dt" and "dl>dd", as that is
currently unnecessary. Authors will then have to adjust their styles,
which could potentially cause unintended side affects to to the change
in specificity of the selectors.

Default Styles
--------------

The default styles for dt and dd elements is not ideal for figures. As
the dd element is usually indented with margin and/or padding, authors
will usually be required to remove this with their own stylesheets,
rather than being able to rely on more sensible defaults. It is true
that eventually UA stylesheets may have special rules for "figure >
dt" and the like, but during the transition period it will be extra
work to remove inappropriate default styles.

Structural Differences
----------------------

Since authors are already familiar with the meanings and structure of
dt and dd when used within dl, their very distinct meanings and
structure when used within the figure and details may be confusing to
authors.

The dl element may contain multiple groups of dt and dd elements,
wheres within figure and details, each element may only be used once.

Within dl, a group must have one or more dt elements followed by one
or more dd elements, whereas within figure, the elements may appear in
any order.

Semantic Differences
--------------------

For the figure element, it's not very intuitive to determine which of
either dt or dd represents the caption and which represents the
content such as an image. Some authors may interpret dd as being the
description, which is what it means within dl, and thus assume that dd
is meant for the caption, and the dt meant for the image or other
content. However, the spec defines these with the opposite meanings,
with the understanding the dt, or title, is a synonym for caption.
Both alternatives points of view make some sense, but neither stands
out as being obviously correct. This will likely lead to a lot of
authors using these elements in reverse.

Unnecessary Elements
--------------------

The dd element is completely unnecessary to distinguish it from the
caption, which has its own element. This combined with the extraneous
div required for IE compatibility adds two extra elements that are not
logically needed. A previous iteration of the spec used the legend
element for the caption, without any special wrapper just for the
content. This model seems much more intuitive and required less markup
to use, and thus easier to get right. Authors who do wish to have an
extra wrapper around the content may use div if they choose, but
should not be required to use it.

When there is no caption, using figure then requires authors to use 3
extra elements, where they should only need one:

<div> <!-- IE legacy workaround -->
<figure>
<dd><img src="image" alt="..."></dd>
</figure>
</div>
This will be unappealing to authors who may, as a result of the
complexity, opt not to use the figure element.

Aesthetics
----------

Aesthetics is not necessarily the sole or most important concern.
However, many authors seem to find the novel use of dt and dd
unpleasant to read. dt and dd are relatively obscure elements, they
are very short abbreviations, and their meaning in context is unclear,
especially in figure. In particular, the following markup seems
mysterious and strikes many as ugly and unintuitive:

<figure>
<dd><img src="image" alt="..."></dd> <dt>A Pair of Cats</dd>
</figure>
This seems to trigger a "yuck factor", and it would be wise to avoid
that. Aesthetics may seem like a relatively unimportant factor;
however, the desire to avoid introducing new elements that led to
dt/dd being used is itself largely an aesthetic concern.


Details
=======

1. Figure content model should be changed to "Flow Content".

2. The third paragraph in the prose (explaining the use of the <dt>
element) should be changed to roughly: "The first child element with
the caption attribute, if any, represents the caption of the figure
element's contents. If there is no child element with the caption
attribute, then there is no caption."

3. The fourth paragraph in the prose (explaining the use of the <dd>
element) should be changed to roughly: "Any additional contents of the
element beyond the caption (defined above), if any, represents the
element's contents. If there are no additional contents, then there
are no contents."

4. "caption" added to the list of global attributes in the "Global
Attributes" section. In its section it should contain roughly: "All
HTML elements in the Flow Content category may have the caption
content attribute set. The caption attribute is a boolean attribute.
When caption is set on an element that is a child of a figure element,
the first such element defines the caption for that figure element. It
is invalid for a figure to have multiple child elements with the
caption attribute set; in such a case, the first such element is the
caption for the figure and the caption attribute is invalid and has no
effect on subsequent elements. The caption attribute is invalid and
has no effect on any element that is not a direct child of a figure
element."


Impact
======

Easy denotation of figure captions, without any compatibility hacks
that may persist long into the future.

~TJ
Received on Wednesday, 13 January 2010 20:57:43 UTC

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