Introductory sections require h2-6, despite not needing a header

Description of problem

<section class="introductory"> errors on a missing first header element. But the documentation clearly says this is not to be linked from the ToC, which means the header-as-section-title concept shouldn't apply.

Adding class="notoc" does not help.

URL to affected spec or repo:

https://w3c.github.io/PNG-spec/#4Concepts.PNGImageTransformation

What happened (e.g., it crashed)?:

All sections must start with a h2-6 element.
How to fix: Add a h2-6 to the offending section or use a <div>.

Occurred 9 times at:

[<section>](https://w3c.github.io/PNG-spec/#4Concepts.Introduction) element
...

Expected behavior (e.g., it shouldn't crash):
h2-6 tag shouldn't be required for introductory sections

Start by comparing the introductory-section guidance in the linked Respec documentation with the validation behavior reported for the PNG specification URL. Trace the check producing “All sections must start with a h2-6 element” and add coverage showing that introductory sections do not require a header; done means the affected sections build without that error while ordinary sections retain the requirement.