HTML API: Support comments created by invalid tag name in tag closers

[dmsnell]

Adds support to the Tag Processor for a form of HTML comment derived from tag closers with invalid tag names. While it's the case for tag openers that an invalid tag name signals the parser to treat the syntax as normal text, if the token encountered is a tag closer then it starts an HTML comment until the first >

For example, <3 cannot start an HTML element because no tag name can start with a digit. However, </3 abides by a different rule. This is structural content and starts a comment. The comment should span until the first > in the text.

​GitHub PR #4256

Side note: can we get a new "Component" in Trac for the "HTML API"?

## Status

• [x] Satisfy linter requirements
• x Create Trac ticket #58007-trac
• [ ] Expand comments, docblocks, add version and Trac ticket refereces
• [ ] Create backport in Gutenberg

## What

Adds support for a few invalid HTML comment forms.

## Why

We want to continue to find and handle edge cases in HTML so that the Tag Processor remains reliable in the face of unexpected inputs.

## How

Captures the patterns described in the HTML specification for parse errors relating to invalid comment construction. Since these are comments it's appropriate to proceed consuming input until the end, and there's no need to look for other tags or attributes in the process.

cc: @adamziel @gziolo

## Status

• [x] Satisfy linter requirements
• x Create Trac ticket #58007-trac
• [ ] Expand comments, docblocks, add version and Trac ticket refereces
• [ ] Create backport in Gutenberg

## What

Adds support for a few invalid HTML comment forms.

## Why

We want to continue to find and handle edge cases in HTML so that the Tag Processor remains reliable in the face of unexpected inputs.

## How

Captures the patterns described in the HTML specification for parse errors relating to invalid comment construction. Since these are comments it's appropriate to proceed consuming input until the end, and there's no need to look for other tags or attributes in the process.

cc: @adamziel @gziolo

[dmsnell]

Thank you @SergeyBiryukov for creating that category, if you did that!

[SergeyBiryukov]

@dmsnell Thank you for suggesting it :)

[costdev]

Moving to 6.2.1 for visibility.

[dmsnell]

Does anyone have any further thoughts on this? If not, would it be fine to merge in?

[costdev]

@dmsnell I just left a few docblock-related suggestions on the PR 🙂

@costdev I think I've addressed all your comments now, thanks. must say I tried to get the docblock ordering and formatting right, but for some reason that's consistently hard to do

[dmsnell]

Thank you @costdev - I believe I have addressed your comments

So there were @costdev - sorry for missing that. I'll double-check once the tests finish again to make sure no remain.

[dmsnell]

Looks like this has been sitting without any further comment or issue for around a week. Is it okay to merge or are there any outstanding concerns?

[dmsnell]

@ockham I've updated the description in the GitHub PR, added comments about the extra syntax forms for which this patch adds support. a note here: it's not possible to visit these new forms, because they do not represent tags, but it's important that the processor doesn't get off-track when it encounters them.

[Bernhard Reiter]

In 55667:

HTML API: Add support for a few invalid HTML comment forms.

• Comments created by means of a tag closer with an invalid tag name, e.g. </3>.
• Comments closed with the invalid --!> closer. (Comments should be closed by --> but if the ! appears it will also close it, in error.)
• Empty tag name elements, which are technically skipped over and aren't comments, e.g. </>.

Props dmsnell, costdev.
Fixes #58007.

[dmsnell]

Thanks @bernhard-reiter!

Committed to Core trunk in `r55667`.

[Bernhard Reiter]

Reopening for 6.2.1 consideration.

[Bernhard Reiter]

In 55668:

HTML API: Add support for a few invalid HTML comment forms.

• Comments created by means of a tag closer with an invalid tag name, e.g. </3>.
• Comments closed with the invalid --!> closer. (Comments should be closed by --> but if the ! appears it will also close it, in error.)
• Empty tag name elements, which are technically skipped over and aren't comments, e.g. </>.

Props dmsnell, costdev.
Merges [55667] to the 6.2 branch.
Fixes #58007.

Backported to Core's 6.2 branch in `r55668`.