We use validators for our themes in order to produce standards compliant websites (HTML, CSS, speed and accessibility).  We ran into this problem earlier in the week though with a theme we inherited from someone else.  The error was that not all of the characters included in the file were UTF-8, the new standard for international character sets. 

I believe that someone had just cut/paste the header values in from an old site and included them directly in the page.tpl.php file.  This isn't unusual when upgrading a site, but certainly demonstrates the importance of validation. As a result, the W3C HTML validator produced this error:

Sorry, I am unable to validate this document because on line 22
it contained one or more bytes that I cannot interpret as utf-8
(in other words, the bytes found are not valid values in the
specified Character Encoding). Please check both the content of
the file and the character encoding indication.

The error was: utf8 "\xE9" does not map to Unicode

Now looking at this file in a text editor line 22 looked just fine (even in vim).

However after forcing the file through the GNU/Linux iconv program and renaming the file:

iconv -f ISO8859-1 -t UTF-8 -c page.tpl.php > page.tpl.php.new

and W3C could validate the page. Loading the whole page in a decent text editor and saving as UTF-8 would also work.