It is hard to say what the physical medium for web design may be, but I feel that it starts with the code. I have a hard time designing a webpage without thinking of the underlying code and working with all the layers (html, css, js...) to do more than reproduce a design, but to actually bring it to life. This often means that things will change, evolve and generally result in a better design and more enjoyable experience. The web space is more constrained by standards and protocol, which mirror the physical challenges of material space, than tools like Photoshop or Illustrator.
Thanks for the comment. Robert