These look like random shapes… until you move the mouse over (or tap) them.
Comment
The effect presumably is related to the fact that the borders of letters vs. background are indistinguishable from the borders of letters vs. ink blot.
Source
Bregman AS (1981) Asking the “What-For” questions in auditory perception. In: Kuy M, Pomerantz J, Perceptual Organisation · Erlbaum