Support HTML in Markdown strings.

[RokeJulianLockhart]

Is your feature request related to a problem? Please describe.
https://mermaid.live/ is unable to render embedded HTML in Markdown strings, per https://mermaid.live/edit#pako:eNoVi0EOgjAQRa9C_pJwgsa44ga6c0wY2kGItDXDNMYQ7m55q7d4b4fPQeAwrfnrZ1Zr7j2lpmKy2YMwtKe0l1GvA-GJDlE08hLqtJ8lwWaJQnBVA-ubQOmoHRfLt1_ycKZFOpRPYJN-4ZdyhJt43eT4A6sNKwU.

Describe the solution you'd like
Render HTML in Markdown strings.

Describe alternatives you've considered
Per mermaid-js/mermaid#5319, mermaid-js/mermaid#5319 (comment) may be relevant, because remediating that reduces the immediate need for this functionality slightly.

Additional context
mjbvz/vscode-markdown-mermaid#246 (comment)