Blotted letters – a Gestalt phenomenon
These look like random shapes... until you move the mouse over them.
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.
Bregman AS (1981) Asking the “What-For” questions in auditory perception. In: Kuy M, Pomerantz J, Perceptual Organisation • Erlbaum