h-card is a microformat class around the mark up of data about identity elements like names, addresses, cities, countries, and often including an avatar or photo. Hovercard is a UI element that creates a visual card when one hovers over a name or similar element that would contain h-card details.
Gravatar serves some of these functions for WordPress from a centralized perspective. The data you would imput there would be wrapped with the h-card class, while Jetpack would give you the ability to display Gravatars as hovercards, so that when you hover over an avatar it will display more detail about the person in a small card-like UI.
For your experimentation purposes, you should be able to use just one post to test against my site. Once you’ve modified your theme, you can simply resend the webmention to my site and that will automatically update your original post. You don’t need to create new posts each time to test it out.