Allow rendering HTML code blocks.

[RokeJulianLockhart]

When a preformatted block (a <pre> or ```) is intelligently considered to contain HTML, or (especially) when a fenced code block explicitly states that it contains HTML, like ```HTML, its content should render if it does not contain disallowed content (per https://github.com/gjtorikian/html-pipeline/blob/11cec3d43cab9f6beddd95bda40ad6ac654fe2f8/lib/html/pipeline/sanitization_filter.rb#L44-L95 ¹). If it contains disallowed code, ideally that code should be (with a notification) merely not executed. However, I would understand if the option to render were disallowed too, if that were too difficult to implement for a reason unbeknownst to me.

I want this because I frequently want to cite a section of a static website in issues so that I and the viewer don't have to view a limited-resolution screenshot. I can already include a code block containing HTML, but a viewer of the issue must manually duplicate it to a local file and execute it, which isn't safe for them, and is a nuisance for us both.

Precedent exists for this in how Mermaid content already is, as https://github.com/github/docs/blob/f8992bc56c56b19e26af839b8fd427824cc99473/content/get-started/writing-on-github/working-with-advanced-formatting/creating-diagrams.md#creating-mermaid-diagrams explains.

Footnotes

1. https://stackoverflow.com/revisions/23978592/1#:~:text=Github%20has%20open%20sourced%20their%20text%20processing%20pipeline.%20The%20definition%20of%20the%20tag%20whitelist%20can%20be%20found%20in%20the%20jch/html%2Dpipeline%20repository. ↩

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