ENH: Display source & render simultaneously

Is your feature request related to a problem? Please describe.

Users can read the source and the render simultaneously; at the same time, in order to learn the symbols.

Describe the solution you'd like

New menu options?

• Menu
  • Show Math As
    • MathML (in a new tab)
    • TeX Commands
  • Math settings
    • Math renderer
      • HTML-CSS
      • Plain Source
      • Plain Source && {HTML-CSS,}
        • MathML
        • TeX

It could say: "show source and render" or something more usable

Describe alternatives you've considered

• toggle back and forth between Plain Source and e.g. HTML-CSS
• click/tap and open each rendered ah expression in a new tab each with "Show Math As"

Additional context

• Jupyter notebooks; which have markdown inputs and variously-mimetype'd obj._repr_mimebundle_() outputs like text/plain and text/html but not yet text/html+rdfa or application/ld+json (which is - as a sidenote - tragic, in terms of symbolic expressions, because these could all be Linked Data expressions that we then also want to review: (source, tex, RDF_with_URIs))