Support `picture` tag for markdown pages

[ilfa]

Hi!

Can you keep picture tag for a markdown pages instead of omitting it?

• Here is the readme on pub.dev without tag
• Here is the same readme on GitHub
• Here is the GitHub documentation page with the recommendations for using picture tag.

[isoos]

@jonasfj: should we add this to package:sanitize_html?

[ilfa]

I can do pr in package:sanitize_html if it will help.

[jonasfj]

Yes, I'm down with adding this to package:sanitize_html, feel free to make a PR.

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But I don't see this in: https://github.github.com/gfm/ OR: https://github.com/gjtorikian/html-pipeline/blob/main/lib/html/pipeline/sanitization_filter.rb

I'm not inclined to support tags not specified in GFM spec or sanitization_filter.rb, maybe github is now using a different sanitization library -- but then we should hopefully be able to find some canonical reference documentation or code for this behavior.

Linking to github user documentation does not seem like a good idea.

In short I would strongly prefer if package:sanitize_html is just an arbitrary set of rules we've "guess" to be okay. I would strongly prefer that the rules in the package are just an implementation of rules implemented somewhere else that we can reference to. Otherwise, next time someone asks for a "feature" we have to discuss it, and worse we might need to investigate the security implications.