Element g:plusone undefined

Element "g: plusone" undefined

If you get the error: element "g: plusone" undefined in w3c validator, it is because XHTML Transitional does not support this tag.

If you want to maintain your markup validation and compliance, change the Google +1 tag as follows:

Change this
<g: plusone></g: plusone>

To this

<div class="g-plusone"></div>