The Mozilla Developer Network has some good HTML reference information but so too does HTML Tutorial.
It is also worth using HTML5test - How well does your browser support HTML5? to see how well browsers support HTML 5.
The <hr> HTML element used to be a horizontal rule, however its meaning is changing and there are ways to do the same in CSS, see <hr>: The Thematic Break (Horizontal Rule) element | MDN for details.
I have also put some handy HTML information and examples at web/html · master · Geoff Lawrence / Geoff Does Stuff · GitLab.
If you want an explanation of "accordions" then Exclusive accordions using the HTML details element | MDN Blog is a great article.