I do think there are strategies and techniques that could be conveyed to help demonstrate website designs & layouts, modifications & improvements, performance & tracking, ads & marketing, beta testing, etc... to others, which would seem to fall under the responsibilities of a webmaster.
Although tags already exist for development and web-development, these would seem to be more concerned with the development process than conducting demonstrations for others, as you noted.
I therefore created a demonstration tag and added it to your question. For its description, I added:
Anything related to the demonstration of a website, including: website designs & layouts, modifications & improvements, performance & tracking, ads & marketing, beta testing, etc...
I've also created a demo tag and added it as a synonym to the demonstration tag, so that if a user types demo they'll be provided with the demonstration tag. The reason I didn't chosedemo as the main tag is because it's often used in reference to software demos, as the demo tag on Stack Overflow indicates.