Lately I’ve been playing with Tooltips a bit.

As in, subclass a button who also shows a tooltip:

desc = ‘To restart the game you can also click ALT+R.’, desc, nil)

Now I compared this to HTML and Javascript. Perhaps you know the fancy
popup boxes you can do in HTML and Javascript?

With colours and a coloured border. And they even move a bit when the
mouse moves.

I was wondering whether there is a very easy way to style those Tooltip
boxes in GTK, perhaps in ruby-gtk3?

HTML, CSS and Javascript gives me a lot of freedom, I feel that quite
often the same is much much harder in Gtk, at least up to version 2 (I
have not played much with GTK 3 yet)