In the JS docs we use <sup> and <sub>. There isn't an equivalent of these in GFM.

How much is it used?

• <sup>: 127 occurrences across 44 files:

https://developer.mozilla.org/en-us/web/javascript/a_re-introduction_to_javascript
https://developer.mozilla.org/en-us/web/javascript/about_javascript
https://developer.mozilla.org/en-us/web/javascript/data_structures
https://developer.mozilla.org/en-us/web/javascript/guide/control_flow_and_error_handling
https://developer.mozilla.org/en-us/web/javascript/guide/expressions_and_operators
https://developer.mozilla.org/en-us/web/javascript/guide/indexed_collections
https://developer.mozilla.org/en-us/web/javascript/guide/numbers_and_dates
https://developer.mozilla.org/en-us/web/javascript/language_resources
https://developer.mozilla.org/en-us/web/javascript/reference/errors/invalid_array_length
https://developer.mozilla.org/en-us/web/javascript/reference/errors/resulting_string_too_large
https://developer.mozilla.org/en-us/web/javascript/reference/functions/rest_parameters
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/array/array
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/array/every
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/array/foreach
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/array/from
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/array/length
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/array/some
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/array/splice
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/bigint/asintn
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/bigint/asuintn
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/bigint
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/date/now
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/date/parse
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/date/utc
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/function/apply
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/function/bind
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/math/exp
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/math/expm1
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/math/hypot
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/math
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/math/pow
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/math/random
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/number/epsilon
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/number
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/number/issafeinteger
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/number/max_safe_integer
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/number/max_value
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/number/min_safe_integer
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/object/tostring
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/parseint
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/string/fromcharcode
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/typedarray
https://developer.mozilla.org/en-us/web/javascript/reference/operators/bitwise_not
https://developer.mozilla.org/en-us/web/javascript/typed_arrays

• <sub>: 14 occurrences across 3 files:

https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/math
https://developer.mozilla.org/en-us/web/javascript/reference/global_objects/string/fromcharcode
https://developer.mozilla.org/en-us/web/javascript/reference/operators/operator_precedence

They're usually used in math expressions in the JS docs, for example:
https://developer.mozilla.org/en-US/docs/web/javascript/reference/global_objects/bigint/asuintn#returns
https://developer.mozilla.org/en-US/docs/web/javascript/reference/global_objects/math/pow
https://developer.mozilla.org/en-US/docs/web/javascript/reference/global_objects/array/length#description

They're sometimes used to write things like 5^th:
https://developer.mozilla.org/en-US/docs/web/javascript/reference/global_objects/array/every#polyfill

What should we do?

Two options:

Option 1: use <sup> and <sub> in the source

This would be OK I think. These tags aren't very widely used, they tend to be used for specialised things, and they are self-contained - they don't pollute big chunks of the page in the way that using HTML <dl> or <table> does. That is, you can write **blah<sup>123</sup>** and it renders fine.

Option 2: use Unicode superscript/subscript characters

You can get superscript and subscript just by using the appropriate Unicode characters: https://en.wikipedia.org/wiki/Superscripts_and_Subscripts_(Unicode_block). It looks like this will work for all the numerics, but I think some of our current usages, like "2^bits" https://developer.mozilla.org/en-US/docs/web/javascript/reference/global_objects/bigint/asuintn#returns, might not be supported (?).