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.