After a futile hours of searching for a standalone HTML+CSS without Javascript that contains both horizontal and vertical tabs, I decided to create one myself.
This is build on code borrowed from:
- Horizontal tabs: https://codepen.io/Wendy-Ho/pen/MWWBvmd
- Vertical panels/tabs: https://codeconvey.com/pure-css-responsive-vertical-tabs/
Not the most fancy looking page, nor customization friendly, but it works:

To-do list:
- highlight a selected vertical tab