Del. Code tit. 22, § 901

Any municipality of this State with a population in excess of 50,000 persons is hereby authorized to levy, assess and collect a tax for general revenue purposes on earned income of its residents and on any income earned within the city by persons not residing within such city but engaged or employed in any business, profession or occupation within such city. Any employer whose business is located outside the corporate limits of a city but who employs persons who are residents of the city shall deduct from such employees’ total income the assessed municipal user tax imposed by said city.