We at OpenStax are trying to add standard meta tags that Zotero will be able to import.
I am using the tags listed here: https://www.zotero.org/support/dev/exposing_metadata
In particular this page (https://rex-web-add-citation-me-xzwsgi.herokuapp.com/books/astronomy/pages/1-3-the-laws-of-nature)
has these tags:

<meta data-rex-page="" name="citation_title" content="Astronomy"><meta data-rex-page="" name="citation_publisher" content="OpenStax"><meta data-rex-page="" name="citation_language" content="English"><meta data-rex-page="" name="citation_date" content="Oct 13, 2016"><meta data-rex-page="" name="citation_author" content="Andrew Fraknoi"><meta data-rex-page="" name="citation_author" content="David Morrison"><meta data-rex-page="" name="citation_author" content="Sidney C. Wolff"><meta data-rex-page="" name="citation_public_url" content="https://openstax.org/books/astronomy-2e/pages/1-3-the-laws-of-nature">

None of them are imported when I use Zotero Connector on the page. My assumption was that tagging this way would be suitable for a generic import. Also possibly significant is that it is not a static HTML page (but I did wait for it to render completely before importing).