I got 2 errors when validating my XHTML strict input type below:
<form action="" method="post" id="frm">
<input type="text" id="searchtext" name="search" value="Suchen" />
<input type="image" id="searchbutton" name="search" src="images/search-icon.png" />
</form>
The errors are:

It happens if you try to put an input directly into a form without a container element, and validate under XHTML 1.0 strict. Put it into a div, and it goes away.
Refer: explanation
why it happens