How to comment/uncomment in HTML code

Depends on the extension. If it's .html, you can use <? to start and ?> to end a comment. That's really the only alternative that I can think of. http://jsfiddle.net/SuEAW/

you can try to replace --> with a different string say, #END# and do search and replace with your editor when you wish to return the closing tags.

Yes, to comment structural metadata out,

Using <script>/* ... */</script> in .html

Comment out large sections of HTML (Comment Out Block)

my personal way in a .html file is opening: <script>/* and close it with */</script>

<script>/* hiding code go here */</script>

Is a workaround to the problem since is not HTML.

Considering your code in .html...

  <!-- Here starts the sidebar -->
  <div id="sidebar">
  ....
  </div>

<script>/*
  <!-- Here starts the main contents pane -->
  <div id="main-contents">
  ...
  </div>

  <!-- Here starts the footer -->
  <div id="footer">
  ...
  </div>
*/</script>

And in a case is HTML inside PHP file using comment tag <?/* or <?php /* and close it with */?> . Remember that the file must be .php extension and don't work in .html.

<?/* hiding code go here */?>

Considering your code in .php...

  <!-- Here starts the sidebar -->
  <div id="sidebar">
  ....
  </div>

<?/*
  <!-- Here starts the main contents pane -->
  <div id="main-contents">
  ...
  </div>

  <!-- Here starts the footer -->
  <div id="footer">
  ...
  </div>
*/?>

Is worth nothing that is not HTML but a common developer practice is to comment out parts of metadata so that it will not be rendered and/or executed in the browser. In HTML, commenting out multiple lines can be time-consuming. It is useful to exclude pieces of template structural metadata containing comments, CSS or code and systematically commenting out to find the source of an error. It is considered a bad practice to comment blocks out and it is recommended to use a version control system. The attribute "type" is required in HTML4 and optional in HTML5.