replace current outline algorithm with one based on heading levels

[stevefaulkner]

This PR is an attempt to bring the HTML spec in line with the decade+ old reality of non-implementation of the outline algorithm in User Agents and the continued presence of misleading content in the HTML spec relating to the outline algorithm.

Notes:

• Removed outline algorithm section
• Updated hgroup definition to reflect what it actually does/is
• Updated numerous code examples to reflect Heading semantics as implemented in User agents
• Updated/removed outline related cruft.

Relates to:

• Suggest adding a warning about outline algorithm #83
• Add heading-focused outlines and :heading #3499
• Obsolete <hgroup> #6462
• CFC: Publish HTML Review Draft as W3C CR w3c/htmlwg#22

The CSS UA styles partial implementation has been left as is
https://html.spec.whatwg.org/multipage/rendering.html#sections-and-headings

Why partial? refer to https://codepen.io/stevef/pen/ZEvmNPg

• At least two implementers are interested (and none opposed):
• The issue at hand is no implementers have shown any interest in implementing for 10+ years
  • Chrome: …
  • Firefox: …
  • Safari: …

---

/dom.html ( diff )
/form-elements.html ( diff )
/grouping-content.html ( diff )
/index.html ( diff )
/indices.html ( diff )
/infrastructure.html ( diff )
/interaction.html ( diff )
/interactive-elements.html ( diff )
/links.html ( diff )
/sections.html ( diff )
/tables.html ( diff )

[JamieKnight]

+1. Within the BBC we have to specifically inform devs that the spec does not represent what is implemented or planned to be implemented. For us at least, this change would have positive real world impact.

[aarongustafson]

This deeply saddens me, but it's the right thing to do.

[yatil]

100% support for this. Maybe removing the outline algorithm makes it possible to find solutions for its use cases that can and will actually be implemented.

[domenic]

As discussed in the thread you link to, we cannot remove the outline algorithm, as documents still have outlines. It instead needs to be based on heading levels, without sectioning complications.

[Alohci]

It's hard to see why it's better to keep an outline algorithm that's totally wrong with respect to the outline that documents actually have, than to simply remove it. There's certainly a case for adding back in an algorithm that describes the true outline accurately, but I don't see why the lack of the latter precludes the former.

[stevefaulkner]

@domenic wrote

As discussed in the thread you link to, we cannot remove the outline algorithm, as documents still have outlines. It instead needs to be based on heading levels, without sectioning complications.

additional commits add simplified version of outline in @annevk's PR #3499

[domenic]

Great! Let us know when the conflicts with the main branch are resolved and then we can work on review.

[stevefaulkner]

@domenic conflicts resolved

[domenic]

Thanks. Could you also make sure the build passes?

[stevefaulkner]

Thanks. Could you also make sure the build passes?

@sideshowbarker said he would take a look at that

[stevefaulkner]

@domenic

Could you also make sure the build passes?

Thanks to @sideshowbarker build now passes

[SaekiTominaga]

Hello, I have a question.

What is the difference between a rank and a level in a heading?

<h1>-<h6> were called "level" in HTML4.

In HTML5 and HTML LS, they are called "rank".
However, the current outline algorithm also uses the term "level".

This time, the term "heading level" has been restored in the Headings and sections but "rank" is still used for heading elements.

[myakura]

Hey @SaekiTominaga ,

IIRC rank is a concept to determine the level of the heading in a corresponding section. They are not the same thing.

Suppose there is a document with three headings:

<h1>h1</h1>

<h2>h2</h2>

<h2>h2</h2>

This would create the following outline:

1. h1
   1. h2
   2. h2

Now, if you change the first h2 to h3, what would the outline be?

<h1>h1</h1>

<h3>h3</h3>

<h2>h2</h2>

It might be surprising but this would yield the same outline (except the text) per the current algorithm.

1. h1
   1. h3
   2. h2

In this case, the rank in the heading of the first subsection is three, but the level of the heading is two.