Description
Over at ruby/ruby, we sometimes have a situation in documentation where certain details may or may not be important to a few readers, but will be irrelevant to many.
E.g., @peterzhu2118 and I are looking at "Hash Equality and Inclusion", whose details may be important to a few but will be irrelevant to many. One of us thinks that info should be at a link; the other thinks it should be included inline.
There may be many other such.
Casting that info as <details><summmary>...</summary>...</details> would be a perfect solution. See <summary>.
Failing that, a weaker alternative would be support for <small>; see <small>.
In either case, support for embedded bold would be important (to give structure without adding headings), along with italic and monofont.