Google provides a script for plusone button. Unfortunately, it's not validated by W3C Validator, you will get 2 errors:
Element name g:plusone cannot be represented as XML 1.0.
Element g:plusone not allowed as child of element td in this context. (Suppressing further errors from this subtree.)
Update on Jan 08, 2012: Google has added a new option named "HTML5 valid syntax" to fix this problem:
To use Google +1 button without w3c errors, There are two ways to fix this problem:
Just use document.write to output the plusone code, for example:
document.write('<g:plusone size="medium" href="https://googleplussearch.chromefans.org" count="true"></g:plusone>');
This way was published by 911web.org, click here to get details.
Solution: Element name g:plusone cannot be represented as XML 1.0