Why without a plugin? Seriously. Someone else has already done the work. You can control font size and color via CSS.
https://wordpress.org/plugins/search/spoiler
The reason I do not a plugin is that plugin is that I am going to implement it thousands of times in my website. And with plugins, even with popular plugin like short code unlimited you do not know what will happen. This is the reason I am looking for a code.
And I will add the the reason that I do not do it with WordPress blocks is that I have vimeo hosted video in my site. And they can have statistics when you embed them with blocks.(because some none tracking parameter the exist in the embed block)
I spend a few days to transfer my website to blocks. And then a few days to bring it beck out of blocks (because of the statistics). I do not to have the same double work process with short code
So it seems to me that you should download several plugins, look at their code, and put together your own version. As you wouldn’t have to offer any sort of options, you could strip it down to something quite efficient.
