Generally speaking, there are no ethical issues with developing an attorney-client relationship via a website or online platform unless prohibited by state regulations or rules governing professional conduct.
In some cases, such as when considering the transmission of confidential information over the internet or other electronic means, lawyers must take extra care to ensure confidentiality is maintained at all times.
As long as law firms take these precautions into consideration when setting up an online office or engaging in client communication via the web, then establishing an attorney-client relationship through this medium should not be considered unethical.