Whitelist <markdown> and <md> as special tags

[ang-zeyu]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• Others, please explain:

Final (hopefully) follow up to #1403 in preparation for markdown-htmlparser patch removal

Overview of changes:

• Whitelist said tags as special tags, hence htmlparser parses its content as text for rendering later, allowing fancy markdown syntax inside
• Add 'invalid html tests'
  • these are first tested with the htmlparser-markdown patch removed with expected failure, then the tags are whitelisted and retested. the htmlparser patch is also reintroduced, for removal in another pr

Anything you'd like to highlight / discuss:
na

Testing instructions:
na

Proposed commit message: (wrap lines at 72 characters)
Whitelist and as special tags

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No blatantly unrelated changes
• Pinged someone for a review!