Semantic HTML5 Backend

[tajmone]

@jirutka's asciidoctor-html5s backend (Ruby) looks promising for creating better HTML5 docs (especially for executable eBooks):

This project provides alternative HTML5 converter (backend) for Asciidoctor that focuses on correct semantics, accessibility and compatibility with common typographic CSS styles.

Local tests and more digging are required, and this thread will be used to aggregate notes, further links and discussions on evaluating the pros and cons, and if there are potential pitfall in interactions with other extensions.