Superscript

Superscript has three jobs and they are not the same job. A footnote marker points at a note somewhere else. An ordinal marks a number as first, second, third. An exponent or a unit is mathematics, where the raised digit changes what the expression means. Only the third one is load-bearing, which is worth knowing before raising anything: the other two are conventions, and conventions vary.

A typeface that takes the job seriously draws the raised characters itself, as superior figures: their own glyphs, a little heavier in the stroke than the text so they do not go thin, spaced for their own size, sitting at a height the designer chose. You ask for them with font-variant-position: super, or with the raw feature through font-feature-settings: "sups", and for raised ordinals with "ordn". See OpenType features for how those switches work in general. The catch is the same one optical size has: most faces do not ship the feature, the declaration is then inert, and nothing on screen tells you which case you are in. Both families this site loads are in the second case, which is why the specimen above shows the declaration doing nothing and says so.

What software does instead is shrink the normal figure and lift it, which is what <sup> gets you by default. It is legible and it is not free. A shrunken figure is thinner than the text around it, so it fades at small sizes; and vertical-align: super grows the line box, so the one line carrying a footnote marker sits further from its neighbour than every other line does, and the vertical rhythm of the column breaks on it. The fix is three declarations: a size around 0.7em, a baseline shift in relative position, and line-height: 0 so the raised box stops contributing height. Keep the <sup> and <sub> elements while you do it, because they carry the meaning that styling does not, and make a footnote marker a real link to its note rather than a raised number that does nothing.

Not everything numeric wants raising. Raised ordinals are a house style rather than a rule: much English publishing sets 1st and 2nd on the baseline, and locales differ, so 1er in French and 1.º in Spanish carry conventions of their own that an automatic “raise the suffix” rule gets wrong. Chemistry and mathematics need both directions, since a subscript (an inferior figure, the same mechanism pointing down) is the only thing telling the 2 in a water molecule from the 2 in a square metre. And if a face has neither feature, one honest option is to leave ordinals alone: a baseline 1st reads better than a spindly raised one.