Ask HN: How do you HTML prototype?

I am looking for a solution that allows me to easily prototype new design on top of a existing site. Currently, I use a bookmarklet that injects jQuery to the site and then I use the Firefox console to write JS that outputs HTML. The result is that I can locally write code and 100% app native look and feel but the approach is fragile and slow.

How do you prototype in HTML but much faster?