Subscript and superscript (subsup)
Scientific and mathematical expressions must be marked so that subscripts and superscripts are programmatically exposed—e.g. using <sub>/<sup> elements or equivalent semantic markup (such as MathML).
Missing sub in scientific expression:
This paragraph has a scientific expression that lacks a proper <sub> element:
Almost every developer's favorite molecule is C8H10N4O2, also known as "caffeine."