Great question — answering as the UphillTech team:
The theme you are going to made has to be developed according to up-to-date coding standards and can be validate : https://validator.w3.org/ You need to use semantic tags : https://www.w3schools.com/htmL/html5_semantic_elements.asp
Full answer by Albedo0: https://wordpress.stackexchange.com/a/308703 (CC BY-SA 4.0).
Need it done for your business? UphillTech.
