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Re: Why embed a fake comment inside an element?

• From: "Liam R. E. Quin" <liam@fromoldbooks.org>
• To: Roger L Costello <costello@mitre.org>, "xml-dev@l..." <xml-dev@l...>
• Date: Fri, 19 Feb 2021 19:07:06 -0500

On Fri, 2021-02-19 at 19:25 +0000, Roger L Costello wrote:
> Hi Folks,
> 
> I am processing a bunch of XHTML documents. Some XHTML documents
> contain things like this:
> 
> <style> 
>    <![CDATA[
>       <!-- 
>           @font-face
>         {font-family:"Cambria Math";
>         panose-1:2 4 5 3 5 4 6 3 2 4;}
>       -->
>   ]]>
> </style>
> 
> [...]

> Question: Why would someone would do this? Is there a benefit to
> embedding a fake comment inside an element?

Short answer, to make sure fragments of script or CSS aren't shown to
users but are still interpreted.


Early Web browsers would display the contents of style elements because
they didn't know about them, and HTML 2 said that the first unknown
element ended the HTML HEAD.

So you had to hide CSS in the document from browsers that didn't know
about it.

THe usual way to do that was to put it in an HTML comment,

<style type="text/css"><!--
  CSS rules here
--></style>

At the same time something similar happened with the script element -
rather than escape every < sign, you could write

<script type="text/JavaScript"><![CDATA[
  for (i = 0; i < 10; i++) {
    for (j = 0; j < 10; i++) {
      if (j < i && randomColours[data[j]]>0) {
        break;
      }
    }
  }
]]></script>

(actually this example didn't work and still doesn't, and is one of
several reasons why CDATA sections were not the right answer, but
that's what people used).

Unfortunately, script elements would also end the HEAD, and people
ended up using a mix of CDATA sections and comments, and you see things
like
<script><!-- /* <![CDATA[ */
 stuff here 
/* ]]> */ --></script>
in a confused attempt to hide the script from older browsers.

So that's likely what's going on here.

With XHTML people had to go one step further, and you will see,
<script><!--//--><![CDATA[//>!--
 . . .
//--><!--!]]>

There's an explaation of this here [1] although incorrectly calls
<!...> an HTM processing instruction.

[1] https://www.sitepoint.com/community/t/cdata-comment/2072/7


-- 
Liam Quin, https://www.delightfulcomputing.com/
Available for XML/Document/Information Architecture/XSLT/
XSL/XQuery/Web/Text Processing/A11Y training, work & consulting.
Barefoot Web-slave, antique illustrations:  http://www.fromoldbooks.org

• References:
  • Why embed a fake comment inside an element?
    • From: Roger L Costello <costello@mitre.org>