Text items in QGIS now support superscript and subscript.
This allows for:
<sup>superscript</sup>
/ <sub>subscript</sub>
components in text, where the text will be vertically super or subscript aligned and automatically sized to 2/3rd of the parent font size. Users can also set a fixed font size for the super/subscript by including the relevant CSS rules in their HTML, e.g. <sup style="font-size:33pt">super</sup>
vertical-align: super
or vertical-align: sub
CSS formatting rules in any other HTML element