Add back minimal styles required for % value height on map elements in the example(s)

[Malvoz]

In #10 (comment) I didn't think it was necessary to add those (and other) non-required styles,
but becase body, html { height: 100%; } is required to set the map element's (or any element really) height to a % value and match the expectation I think it's a good idea to note that with the minimal styles:

<style>
  /* These styles are required if you wish to use
  a % height value on the mapml-viewer element. */
  html,
  body {
    height: 100%;
  }
</style>

[Malvoz]

Or this could be part of #26.

Edit: Potentially both.