Thanks, in stackexchange sites, they use a pair of CSS properties "visibility: hidden;" and "visibility: visible;"
I think I can use a different pair "display:block" and "display:none" as well.
But "hover" is not a good trigger, it won't work in mobile view. Stackexchange sites use some kind of a javascript that changes the class of the spoiler area.
I think maybe I can use a pair HTM5 tags <summary> and <details>, at least I use it in some non-Q2A pages.