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Defining "Heading" in HTML and PDF
| Does "H2" in HTML mean the same thing as "H2" in PDF? Don't assume so. |
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Thursday, April 19, 2012 Return to Logical Structures
The term "Heading" has substantially different meaning depending on the technology context. This post explains that the "heading" concept in HTML 4 differs critically from the "heading" structure concept in PDF with profound implications for how WCAG 2.0 applies to the Portable Document Format.
I've been spending time recently doing intensive research on WCAG 2.0 and how it applies to technologies that are not inherently "of the web". As always, my focus is on PDF.
Last week I wrote an article discussing whether headings are for navigation or decoration which concluded that WCAG 2.0's mission could only be served in PDF if headings were logical, which boils down to the simple statement that skipping heading levels (H2 to H5, for example) is prohibited.
My argument, in brief, was that just as WCAG 2.0 requires attention to valid structural markup for lists and regulates how authors can use color (for example), in PDF at least, so too must authors provide "logical heading levels" in the tags tree if they provide "structure" elements at all. Neither PDF nor PDF/UA require the author to provide structure elements if the content does not include structure.
Heading levels in PDF that do not reflect the a meaningful and navigable document structure cannot comply with WCAG 2.0, Success Criterion 1.3.1 or 2.4.3, both Level "A".
As I continued to look at this question I realized that I was actually dealing with a more basic point of confusion: the casual assumption that an "<H2>" tag in PDF actually means the same thing as "<H2>" in HTML.
Au contraire.
Headings in HTML and PDF, a Comparison
| Specification | Definition of <H> and <H1> through <H6> |
|---|---|
|
Unambiguously "importance", use as section heading is discussed. |
|
|
PDF (ISO 32000-1:2008 14.8.4.3.2, Table 335) Official ISO version Free authorized download from adobe.com (pdf) |
Structure Type "H" "A label for a subdivision of a document’s content. It should be the first child of the division that it heads." Structure Types "H1-H6" "Headings with specific levels, for use in conforming writers that cannot hierarchically nest their sections and thus cannot determine the level of a heading from its level of nesting." |
|
Unambiguously "section heading" (no mention of "importance"). |
HTML 4, HTML 5 and PDF each have a distinct definition of "heading", but HTML 5 and PDF are far more similar than HTML 4 and PDF.
Conclusion
By definition, in PDF, the "H" and "Hn" tags denote "subdivisions" of content (we'll not get into "strongly" and "weakly" structured for now). "Subdivision," of course, may or may not mean the same as "importance," depending on the specific document, but either way, the concept is distinct from the HTML 4 notion of "heading".
What logically and invariably follows
- WCAG 2.0 Technique H42 cannot be applied to PDF. This technique is specific to HTML 4.
- Eric Meyer's blog post (referenced in Technique H42) is HTML 4 specific and likewise does not apply to PDF. What's more, this post simply represents his "gut feeling" (he says it himself), hardly grounds for formal advice on conforming with normative text!
- WCAG 2.0 PDF Technique 9 (PDF9) states "In some technologies, headings are designed to convey logical hierarchy". This is misleading in a PDF-specific technique. According to ISO 32000-1:2008, PDF headings ONLY convey hierarchy: that is their function.
Key Takeaway
WCAG 2.0 informative text cannot arbitrarily redefine terms in existing normative standards (ISO 32000). "Headings" in PDF are what ISO 32000 says they are, period.
How to Achieve Success Criteria 1.3.1 and 2.4.3 in PDF
In PDF, the standard structure types are the AT-operable structure, content relationship and navigation mechanism of choice.
In PDF, Headings (H, Hn) are "structure elements", not indicators of "importance", although the two may overlap.
WCAG 2.0 Success Criterion 1.3.1 (Level A) requires "information, structure and relationships" be "programmatically determinable".
WCAG 2.0 Success Criterion 2.4.3 (Level A) requires in the case of sequentially navigable content that "navigation sequences... [are] in an order that preserves meaning and operability."
WCAG 2.0 Success Criterion 2.4.6 (Level AA) and SC 2.4.10 (Level AAA) identify and specify Section Headings in a manner that is compatible with PDF/UA.
We can deduce that logical heading levels are Level A (in the cases of SC 1.3.1 and SC 2.4.3) and Level AA (in the case of SC 2.4.6) requirements for PDF files that use headings.
In practice this requirement will tend to matter on longer (>5 pages, perhaps?) documents rather than shorter ones. For short documents, headings used without attention to the logical ordering of heading levels is usually sufficient to provide basic navigation to the AT user.
PDF implementers refer to ISO 14289-1 7.4 for normative text detailing requirements for meeting WCAG 2.0 Success Criterion 1.3.1, 2.4.3, 2.4.6 and 2.4.10 in PDF documents that include structure elements.
REMINDER OF MY USUAL DISCLAIMER: This post represents my personal view and is not intended to represent in any way the official views of the US Committee for PDF/UA or ISO 32000.
Comments [ Add a Comment ]
Sailesh,
Thank you for your comment. I'm sorry about the captcha - we've noted it in our accessibility logo and are discussing it with the vendor.
Here's my answer.
In HTML it's certainly provided that one might use headings for structural purposes, but headings aren't defined that way - that's my point. In the table in which I discuss the various definitions I do say, for the HTML 4 definition "...use as section heading is discussed." I could make that stronger. The point, however, is that HTML doesn't state that headings are for structure - it provides that structure is one excellent use of headings, but it''s hardly more definitive than that.
Regarding your "sale ends" example, I agree with you that it might or might not be marked up as a heading for a variety of reasons. PDF/UA would not require anything of it based on styling, only that it's given a logical function in the document.
If the document does not conform to PDF/UA because someone decided to tag the "Sale" text as an H2 following an H4 and before another H4, for example, I'm not going to claim that this choice, even though it violates PDF/UA, makes a one-page PDF file un-navigable.
All I would say is that in such a case an AT user could not expect to rely on the PDF's headings for navigational purposes.
What if that page is an advertisment in the middle of a 500 page catalog of 15,000 parts, and thus, 15,000 headings? Headings play a critical role in content navigation.
The point of PDF/UA is simply that strict conformance requires that IF you use heading levels THEN they must be logical to comply with section 7.4 of the standard. That is not to say that if they aren't logical then the document might not be reasonably accessible. It's simply that in such a case users will have no assurance that heading levels will offer a logical navigational experience.
Thanks very much for the input!
Duff.
Hello Duff,
I looked at the HTML 4 specs and also Eric Myer's post you reference.
But I believe headings refer to hierarchical topic markers in HTML 4 too.
In that section you reference 7.5.5 of HTML 4, I believe the word 'important' should not be pulled out of context to imply any important text can be marked up as a heading.
From HTML 4 specs:
"A heading element briefly describes the topic of the section it introduces. Heading information may be used by user agents, for example, to construct a table of contents for a document automatically.
There are six levels of headings in HTML with H1 as the most important and H6 as the least."
For instance text like 'Sale ends tomorrow April 26' on a page may be important but does not fit the hierarchical heading structure ... so I guessed it cannot and should not be marked up as a heading although it may be colored / styled to be eye-catching.
However, from a usability and business perspective, one could argue it could be marked up as a heading and I'd agree but not marking it up as one would not mean it is a WCAG 2 failure
I wanted to reply via your blog but could not get to the rich edit box with JAWS and the CAPTCHA turned me off.
Thanks,
Sailesh Panchang
[Posted on Sailesh's behalf by site admin]