HTML

The Mozilla Developer Network has some good HTML reference information but so too does HTML Tutorial. MDN also some some great learning material, like 2. Semantic HTML | MDN Curriculum.

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.

Forms

It is always good to have an interactive page where input is given, so check out TypedArray - JavaScript | MDN for help with forms.

Images

There are lots of ways to work with images, but getting correctly sized, responsive images is not always easy. If you read : The Image Embed element - HTML | MDN you will see that if you set loading="lazy" this will delay image file downloading, if you use this with sizes="auto" and images with size information, you don't need lots of logic.