A HTML prototype is fantastic for usability testing. Once you have designed your prototype, put it in front of users to get valuable feedback and iterate on the design.
A HTML prototype is more versatile than using mockups for testing, as you can simply send someone a link to view it in their browser as an actual, working interface.
Clients love seeing HTML prototypes early in the design phase. It gives them the chance for feedback on something that feels real.
The prototype is also useful as an internal tool for bringing the team to a shared understanding of a feature, which is essential for a smooth design and development process.
A HTML prototype should never be considered production-ready. The prototype is still in the design phase, so you can take shortcuts with the HTML and CSS to get it done quicker.
The point is to illustrate an idea as best you can. Production ready code happens when the feature is being built.
That being said, front and back end developers may find the prototype to be a very handy reference point for writing the production code.