What is the ruling on the money taken by the owner of an Islamic website from the operational and developmental donations made to the site? Is he obligated to inform the donors of this? And what is incumbent upon him if that becomes difficult?

The money must be spent according to the donor's condition. It appears the donor gave the money to the questioner by way of agency, so the questioner is not permitted to take anything for himself without the donor's permission. The questioner's statement that the website requires effort and time does not alter the ruling, because he is an agent, and an agent cannot transact with himself except with the permission of his principal.

Therefore, he must repent to Allah and guarantee the money he took for himself by returning an equivalent amount to be spent on operating and developing the website, unless he informs the donors of what happened and knows that his spending of the money on himself does not deviate from their intention, considering the effort and time he expends.