Sectioning

<body>defines the body section which contains the content of an HTML document. All subsequent tags in this subsection go into the body section.

<hr /> displays a horizontal line.

<br /> inserts a line break.

<!-- (comment xxx) --> define a comment. A comment is used to explain a piece of code.

The following tags on this webpage allow the contents to be formatted using CSS. Correctly using an element type makes the code more readable and enhances SEO.

<main>, <header>and<footer> define the main content, the header and the footer respectively.

<aside> defines a section that does not belong to the main flow. It contains side information like an explanation box or an advertisement.

<nav> groups navigation links.

<section> denotes a section.

<div>denotes a generic block of content. <main>, <header>, <footer>, <aside>, <nav>, <section> can be regarded as a <div> element with a special purpose.

<p> denotes a paragraph on a new line.

<span> provides a hook to group inline elements and format the text within with CSS, without starting a new line.

<article>contains an article. The self-contained content could exist independently of the rest of the content.

<blockquote> indents a section that is quoted from another source.
...... cite=URL
<cite> cites the title of a piece of creative work.

<details>represents a widget from which the user can obtain additional information, by clicking the widget for example.
.......open: makes the details visible
<summary>specifies a summary inside <details>.

The <dialog> element represents a dialog box or other interactive components, such as a dismissable alert, inspector, or subwindow. (see the last example here)
......open: indicates the dialog box is active and can be interacted with.

There should be only one <main> element without the "hidden" attribute.