In XHTML, cannot use "<" in script

I want use XHTML to prevent forgetting close tag, cause the element set as display:none. I want aslo input script in the document. When I put i < arguments.length, the browser notifying "StartTag: invalid element name". And then I tryied to put the script as external, yet the result is same. Why inside script < (lower than) also read as "starttag"!

Anyone have idea?