HTML comments can be used to leave notes to yourself or other developers about a specific point in code. They can be initiated with <!-- and concluded with -->, like so:

<!-- I'm an HTML comment! -->

They can be incorporated inline within other content:

<h1>This part will be displayed <!-- while this will not be displayed -->.</h1>

They can also span multiple lines to provide more information:

<!-- This is a multiline HTML comment.
  Whatever is in here will not be rendered by the browser.
  You can "comment out" entire sections of HTML code.
-->

However, they cannot appear within another HTML tag, like this:

<h1 <!-- testAttribute="something" -->>This will not work</h1>

This produces invalid HTML as the entire <h1 <!-- testAttribute="something" --> block would be considered a single start tag h1 with some other invalid information contained within it, followed by a single > closing bracket that does nothing.

For compatibility with tools that try to parse HTML as XML or SGML, the body of your comment should not contain two dashes --.