<sub> and <sup> tags can be treated as variants by Firefox (June 2011)

Using Firefox 5 and the Mozilla CSS Extension -moz-font-feature-settings

This html code

<p>NO₂ Aₙ Mᵐᵉ Bib&#xf70d;&#xa7f3;ᵘᵉ Nᵃˡᵉ 1ᵉʳ 2&#xf70d;ᵐᵉ 1ˢᵗ 2ⁿᵈ 3ʳᵈ</p>

gives this result

NO₂ Aₙ Mᵐᵉ Bibꟳᵘᵉ Nᵃˡᵉ 1ᵉʳ 2ᵐᵉ 1ˢᵗ 2ⁿᵈ 3ʳᵈ 4ᵗʰ

And this html code

<p>NO₂ A_n M^me Bib^èque N^ale 1^er 2^ème 1^st 2^nd 3^rd 4^th</p>

should give the same result

NO₂ A_n M^me Bib^èque N^ale 1^er 2^ème 1^st 2^nd 3^rd 4^th

If the second result differs from the first, it is because you do not use Firefox 5!