All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad
is His slave and Messenger.
Working in website design and maintenance is, in principle, permissible and there is no objection to it. However, it is not permissible to provide such services to entities whose primary and majority activities involve prohibited matters, such as interest-based banks and similar institutions, because this constitutes assisting them in sin. Allah, the Exalted, has prohibited this by saying (which means): {And do not cooperate in sin and transgression. And fear Allah; indeed, Allah is severe in punishment} [Quran 5:2]
As for companies whose core business activities are permissible, it is allowed to deal with them in designing and maintaining their websites, provided that the design is not specifically intended for, or dedicated to, a prohibited service. The same ruling applies to hotels: there is no harm in designing websites for them, since the basic nature of their services is permissible. However, if you are asked to create a specific design for a prohibited service or to promote something unlawful, then it would not be permissible to do so.
Allah Knows best