Are nested HTML comments possible?

Question

as per the title; is it possible to have nested comments in valid HTML? see the example below...

<p>some text</p>    <!-- comment 1      <p>commented out html</p>      <!-- comment 2        // are nested html comment allowed?      end of comment 2 -->      <p>more commented out html</p>    end of comment 1 -->  <p>some more text</p> 

It appears not, does anybody know how I could get nested comments to work?

Answer 1

When you nest a comment, replace "--" with "- -". When you un-nest, reverse the procedure. It's not the <!-- that is forbidden but the --.

Example:

<!-- some stuff <!- - some inner stuff - -> <!- - a sibling - -> the footer --> 

Answer 2

TL;DR: Unfortunately, no, it's not possible (and never will be).

Short answer:

An HTML comment is not quite what many think it is. HTML is a form of SGML, in which comments are delimited by pairs of double-dashes (-- … --).

Thus, any pair of a double-dashes inside a pair of angle brackets with an exclamation point after the opening bracket (<! -- ⋯ -- >) is a comment. The spec says it better than I can: http://www.w3.org/TR/html4/intro/sgmltut.html#h-3.2.4

This is why comments like this one (which we've all likely done one time or another) are a bad idea:

<!-- ------------------ HEADER BEGINS HERE -------------------- -->

Truth: I am too lazy to tell you how many comments are represented by the above tag pollution, but it's at least 10.

I got less lazy: This so-called "comment" actually consists of 10 comments, three words outside any comment (i.e., just bad SGML), and the beginning of a comment that is not terminated. It's a real mess:

<!--1----2----3----4----5-- HEADER BEGINS HERE --6----7----8----9----10-- -->

Of course, it's not quite that simple, due to differences in how each browser chooses to interpret the spec.

Here's an excellent article that explains it:

    http://weblog.200ok.com.au/2008/01/dashing-into-trouble-why-html-comments.html

Long answer: Why we get it wrong

Most of us who grew up with HTML (without delving into the SGML that underlies it)) have come to believe that the string <!-- begins a comment, and the string --> ends a comment.

Actually, <! and > delimit an SGML declaration within your HTML document, such as the DOCTYPE declaration we've all seen at the top of our pages. Within an SGML declaration, comments are delimited by double-dashes. Thus, the HTML comment

<!-- this is a comment -->

which most of us would believe is parsed like this <!-- this is a comment --> is actually parsed like this:
<!-- this is a comment -->. It is an SGML declaration that is empty except for a comment.

Because HTML is a form of SGML, this "comment-within-a-declaration" functions as an HTML comment.

Out of interest, here's a chunk of pure SGML that shows comments functioning as they were intended in SGML: this attribute list definition contains a comment on each line:

<!ATTLIST LINK   %attrs;                              -- %coreattrs, %i18n, %events --   charset     %Charset;      #IMPLIED  -- char encoding of linked resource --   href        %URI;          #IMPLIED  -- URI for linked resource --   hreflang    %LanguageCode; #IMPLIED  -- language code --   type        %ContentType;  #IMPLIED  -- advisory content type --   rel         %LinkTypes;    #IMPLIED  -- forward link types --   rev         %LinkTypes;    #IMPLIED  -- reverse link types --   media       %MediaDesc;    #IMPLIED  -- for rendering on these media -- >