Clean up HTML output & improve accessibility

[marcelltoth]

Summary

With #109 merged, we now have the option to output much nicer HTML than we do right now.

This would help with both the new product initiative of promoting accessibility across the product, and with getting the tests in shape. (My prior attempts like #112 turned out too difficult because the structure was really not testing-friendly. It could be now.)

Tasks

• Use proper nested lists instead of divs everywhere.
• Apply proper roles and aria-attributes for the tree. Example here.