Ask HN: What should be considered when starting a web development project?

I am experienced in software development, mostly on embedded and desktop platforms. I have a client who has requested me to develop for them a web application. This will be my first web application that will be deployed. I have in the past developed web apps but it was for learning purpose and were not deployed in the wild. So I would like to know, when starting a web app/development project what minimum(or maximum) things should be taken into consideration from the point of view of security, scalability and maintainability so that there is minimum disruption as the project grows?