HTML Tidy question

With the development we do we use placeholders that are replaced with  
html, javascript, etc.  They are in the format of <ARCplaceholder  
name="FolderTabs">.  They can be on a line by their selves or within  
other html like <td><ARCplaceholder name="FolderTabs"></td>.  There is  
no end tag.  I can not figure out how to get HTML Tidy to ignore these.  
It gives a warning for every one:  

line 7 column 1 - Warning: <arcplaceholder> is not approved by W3C  
or  
line 7 column 1 - Error: <arcplaceholder> is not recognized!  
line 7 column 1 - Warning: discarding unexpected <arcplaceholder>  

Any help would be greatly appreciated.  

Serkant KARACA
Senior Software Engineer
Ankara/Turkey
http://yagmurzamani.blogspot.com
http://www.internetteyim.net