InvisiText lets you hide text with pure CSS.
You can play around with Invisible Ink here: https://codepen.io/desperate-coding/pen/qXMNwx
See this website.
Thanks to Darryl Huffman who made a very nice JS library to make invisible ink. That first got me wondering whether there was a CSS-only solution.
More thanks to Adam Ruf who experimented with the invisible ink idea. This gave me the idea first to add the hovering / clicking feature.