Markdown heading does not correspond to right HTML Section Heading elements

[kittauri]

Describe the bug
A single # in the textblock generates a h2 in HTML. It is not a direct bug but a wrong implementation of Markdown, as it is intentional in there.

volto/packages/volto-slate/src/editor/plugins/Markdown/constants.js

Lines 23 to 31 in 467e0a3

	export const autoformatRules = [
	{
	type: H2,
	markup: '#',
	},
	{
	type: H3,
	markup: '##',
	},

I know that you want to limit titles to an h2 heading, but a user may expect to have an h1 with single #.

To Reproduce
Steps to reproduce the behavior:

1. Create a heading in a text field with a #
2. Inspect the Element with the Developer Tools
3. You see the mismatch between documentation, the generated HTML and the specification

Expected behavior

A single # should generate a h1 and two # should generate a h2 etc.

As stated in the specification:
https://spec.commonmark.org/0.31.2/#atx-heading
or here
https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#headings

Screenshots

Software (please complete the following information):

• OS: Linux
• Browser: Chrome (Version 131.0.6778.264 (Official Build) (64-bit))
• Volto 18.4.0
• Plone 6.1.0b2
• plone.restapi 9.9.0

Additional context
There is the intention to prevent the editor from creating to much h1 headings so it is not clearly a bug but a feature, but it is wrong implementation.

[davisagli]

This is by design. h1 is reserved for the heading created by the title block, and there should not be more than one h1 on a single page.

[erral]

Then perhaps we should not allow # but use ## for h2 and ### for h3, right?

[davisagli]

@erral That does sound like it would be a bit more consistent with what is actually rendered.