The HTML comment tag !-- -- is used to insert comments in the source code of the page. Anything placed in the space between the !-- -- tags is considered a comment and is not displayed in the browser.

					<!-- This is a comment where we make notes about our code -->
<p>This is a paragraph.</p>

Browsers will not display comments on the page, but they can still be seen in the source code. This is most often used to write notes about the code itself for future reference. However, it can also be used to keep something from showing on the page temporarily, which can then be uncommented at some point and used again without the need to re-write the code.

					<!-- <p>This is a hidden paragraph.</p> -->
<p>This is a <!-- partially hidden --> paragraph.</p>

The comment tag can also be used across several lines of code to “comment it all out”.

<h1>This is our hidden heading</h1>
<p>And this is our hidden paragraph</p>

A few things about HTML comments:

  1. A comment cannot be placed inside another comment
  2. The double-dash sequence cannot appear inside the comment except as part of the opening and closing tag
  3. The opening and closing tags of a comment cannot have spaces separating their characters; having a space directly before or after is fine