(this entry is a bit of a mess, just posting it for reference now, might get around to expand the topic at a later time)
According to the internet, IE has issues with the UTF-8 version of subscript 2: CO₂ … which unfortunately turns out to be correct, also in IE8. IE just displays an outlined square.
when writing m², you can either use the entity
<sup>2</sup>. There is no entity for subscript-2 and the UTF-character
₂ doesn’t work in IE
Without some extra CSS in the document they differ a bit,
² being more typographically correct. <sup> and <sub> elements messes up the line spacing.
Examples (entities vs. sup/sub): m² vs. m2 and CO₂ vs. CO2