Thanks for the suggestion!
Putting a zero width space between the colons does indeed seem to work for the standard cloze deletion cards.
I should have mentioned that i also want to make a second cloze deletion card type, where i’m prompted to type in the correct answer.
When comparing the given answer with the enclozed text, Anki recognizes the zero width space as a character and marks the given answer as wrong, if you don’t type a space between the colons.
You could obviously just ignore this “false negative”, but its not ideal. So in this respect it seems like the <span> element is the better solution.