When is a CDATA section necessary within a script tag?

Question

Are CDATA tags ever necessary in script tags and if so when?

In other words, when and where is this:

Answer 1

Basically it is to allow to write a document that is both XHTML and HTML. The problem is that within XHTML, the XML parser will interpret the &,<,> characters in the script tag and cause XML parsing error. So, you can write your JavaScript with entities, e.g.:

if (a > b) alert('hello world');

But this is impractical. The bigger problem is that if you read the page in HTML, the tag script is considered CDATA 'by default', and such JavaScript will not run. Therefore, if you want the same page to be OK both using XHTML and HTML parsers, you need to enclose the script tag in CDATA element in XHTML, but NOT to enclose it in HTML.

This trick marks the start of a CDATA element as a JavaScript comment; in HTML the JavaScript parser ignores the CDATA tag (it's a comment). In XHTML, the XML parser (which is run before the JavaScript) detects it and treats the rest until end of CDATA as CDATA.