There Is No Document Outline Algorithm
I figured I would state the entire argument in the title. After all, as of this writing and the last seven-plus years, the statement is accurate as far as the browsers are concerned.
I am penning this as sort of a follow-up to my post from 2013, The Truth about “The Truth About Multiple H1 Tags”. Even after that post helped kick off an update to the W3C HTML5 specification, it is not reflected in current tutorials and informative pieces.
Bad Information Persists
The appeal of ignoring heading levels for developers and authors is pretty compelling when you do not know how or where that content will appear (or it appears in many different locations). In particular, the all-
<h1> approach appeals to many for its simplicity.
It makes sense why a developer might see this advice, or hear about it through misinformed articles, and never look back. This advice is a free pass. Many content authors don’t even know where their content will appear, making an all-
<h1> approach feel like the safe approach.
Unfortunately, despite all the activity in the standards world along with the lack of activity on the part of the browsers, many developers continue to be unaware that this imparts no benefits to users and even harms many of those users. I run into this repeatedly when I answer questions on Stack Overflow, when I talk to developers in real life, and even from generally trusted outlets:
This was part of the spec, and it was “revoked”, which is not a nice thing to do. And it was revoked not because they considered that it was a bad idea, but because of screen readers not implementing it correctly.
This is a common position, captured succinctly in this one example.
Disregarding the fact that it was never part of the final W3C spec, that the spec had a warning for three years, that nobody considered the algorithm a bad idea, that screen readers had nothing to do with it, and that browsers not implementing it is different from correctly implementing it, there is one statement that belies the issue at hand.
Not a nice thing to do is a value judgment. It presumes that the specification’s primary benefactors are developers when in reality it is about users. It also presumes that it is acceptable to give developers advice (that harms some users) that has never been supported.
Like it or not, browsers are not moving on this feature and citing the purely theoretical document outline does nothing to move it forward. We as developers need to resolve this while still making it easy for content authors.
Update: February 13, 2017
There is a new issue opened against the W3C specification to try to understand how the outline algorithm is supposed to work so a polyfill can be created. This is sometimes a first step to getting support built into browsers. Read more at the issue, Update outline algorithm #794.
One Alternative (in Two Parts)
The average web developer would rather not have to think about mapping the appropriate heading level for every potential re-use of content. Authors should not have to think about it at all.
Way back in the early oughts (actually, 1999–2000) I wrote a CMS (Content Management System) based on delimited text files. It was a lark. I wanted to teach myself some programming skills and my brother needed a mini-CMS while he was overseas.
I quickly ran into the heading issue that HTML5 tried to solve — sometimes his content would be re-used elsewhere in the layout, and the headings would not make sense anymore. But I solved it. I solved it without any fancy frameworks or libraries or HTML5 retooling.
Every content container carried a variable (this was all server-side code). That variable was a number reflecting its nesting level on the page. That number was then used to replace the number in any
<h#> levels in the content (the content was chunked enough that there was not more than one heading).
I carried that technique forward into projects on much beefier CMSes and never had to worry about training authors how to manage chunked content on their home pages (and similar chunked pages). The move to HTML5 never made me consider an all-
<h1> solution, partly because I knew the outline was not supported.
Since so much of the content on modern sites comes in via client-side scripting, the code would simply need to be updated to run in the browser — this assumes you don’t mind offloading simple processing to thousands of users across uncontrollable run-times. But then if you are relying on client-side scripts to render a page you have already made your decision.
The following embedded code shows an HTML document that uses an all-
data- attributes on the sectioning containers, I re-write the
<h1>s to reflect a document outline appropriate for this content. I use some CSS generated content to include the heading level after the text of each heading so you can easily see which is which. If the script does not work, you will see black headings sans parentheses for all.
Conceivably you can let your authors continue an all-
<h1> approach, while your templates just tweak the structure based on attributes you embed in the layout.
See the Pen Dynamic Heading Level Demo by Adrian Roselli (@aardrian) on CodePen.
You can see (steal, fix) the script at CodePen directly, or you can view it as a full page and make sure your assistive technology (such as a screen reader in this case) can navigate the corrected heading structure as you expect.
We can work to get browsers to support another new element, the
<h> element (proposed in April, which was probably based on Gez Lemon’s 2004 suggestion). Browsers would still need to implement some sort of document outline algorithm, but in this case a new element means no need to rewrite existing
That will require the developer community to come together as it did for the
<picture> element. It can be done, it just requires some effort.
Update: January 18, 2017
While the issue opened last April has since been closed (since it was about a language change in the spec), a new issue was opened specifically for adding an
The statement in the title of this post is not new. It has been discussed for at least three years in standards bodies. It has been ignored by browsers for longer (more than seven years), though the browser bugs linked at the end are only a couple years old. Anyone who claims this is a recent change has not confirmed that with the W3C specification in two years.
The following links are just evidence I have needed to provide repeatedly to demonstrate these points. I guess they are more for me to easily reference from future Stack Overflow answers.
- In October 2013, Steve Faulkner (an editor of the HTML5 specification) wrote a post, The HTML5 Document Outline, explaining that it does not exist outside of the spec.
- In January 2014, discussions started on the W3C HTML Working Group mailing list.
- On January 25, 2014, Mozilla Developer Network updated its advice, acknowledging there is no outline algorithm in any browser.
- In March of 2014, a W3C HTML 5 bug was opened to “modify required heading mappings to reflect reality.”
- As of April 17, 2014, a warning was added to the W3C draft HTML5 specification explicitly stating there are no implementations of the outline algorithm.
- On October 28, 2014, the W3C HTML5 specification was officially released as a W3C Recommendation. It included the warning about the document outline.
- On September 1, 2015, Steve Faulkner opened an issue with the WHATWG HTML5 specification suggesting a similar change. That bug is still open.
- As of January 22, 2016, Steve Faulkner re-opened the discussion with the W3C HTML5 specification to see if there was some way to reflect reality.
- On April 7, 2016, Chaals opened the issue Do not recommend using nested sections with h1 against the W3C HTML5 spec (which is also where a
<h> element was proposed).
- As of June 9, it was removed from the HTML 5.1 draft specification.
- On June 16, 2016, both the W3C HTML Checker and the W3C Markup Validation Service were updated to issue warnings, but still let you see the ideal outline (if it was supported).
- Chromium bug to support the outline algorithm (still open).
- Firefox bug to support the outline algorithm (still open).
- WebKit bug to support the outline algorithm (still open).
- Internet Explorer bug to support the outline algorithm (still open).
- Added January 17, 2016: Add element #774 formally opened against the spec, tweeted by W3C Web Platform WG.
To recap, the Document Outline Algorithm was never a recommendation in a final W3C spec. There was a warning explicitly against authors relying on it, though the outline language was retained for browsers to understand how to implement support (eventually).
Regardless of whether you like the idea of the document outline algorithm, it does not reflect reality since no user agent supports it.