Here are the main reasons why commented code is a code smell:

• It always raises more questions than it gives answers
• Everybody will forget very quickly how relevant the commented code is
• This is distraction when going down the code as it stops the flow of eyes
• It is a bad SCM engine : Subversion, CVS and Git are really more trustworthy !
• The simple fact of understanding why code was commented out in the first place can take a lot of time

Noncompliant Code Example

<p>
<!-- Non-Compliant -->
<!--
<b>Hello</b>,
<i>world</i>!
-->
...
</p>

Compliant Solution

<p>
<!-- Compliant -->
...
</p>