gleb writes:

Two years ago I grew tomatoes on the bathroom windowsill. The seedlings
soon developed some leaves which became papery and white and the plants
had to be discarded. Last year I disinfected the windowsill and tried
again, same result. This year I am growing my tomatoes elsewhere but am
growing celery on the bathroom windowsill and the seedlings are
suffering the same fate. Does anybody know what is wrong and how to fix
it?

Too much light for young plants?
Is this a south facing window?

Otherwise inspect top and bottom of leaves with magnifier.
Look for fungus or insects.

--
Dan Espen