I don't think Martijn is asking for html. A normal text field should accept normal unicode text which includes characters like: Đíť ìš éèñ ëņċøďıńğŧęŝţ .. ₀0⁰₁1¹₂2²₃3³₄4⁴₅5⁵₆6⁶₇7⁷₈8⁸₉9⁹
So... just find the subscript 2 on your keyboard and enter it. All is fine.
I guess if you use the Rich Text Widget you should be able to do it. This is H₂O
You may need to use the full set of buttons in the Rich Text Editor so you can edit the HTML directly
A list of codes can be found here
I think it is only possible when using html ( tag).
This can be done with widgets from the app-store i guess. For e.g. the microflow-label widget or perhaps the html widget.
I'm not sure if it entirely fits your need, but you can give it a try.