I have the google+ button being rendered on my site as follows:
In the JS:
var po = document.createElement(\'script\'); po.type = \'text/javascr
In addition to Jenny's answer you can also use a HTML5-valid +1 tag by setting the class attribute to g-plusone, and prefixing any button attributes with data- such as callback
<div class="g-plusone" data-callback="myCallback" ></div>
From https://developers.google.com/+/web/+1button/
You can add a JavaScript callback using a callback attribute to your +1 button markup. Supply the name of a function that resides in the global namespace. It will be called when a user clicks on the +1 button.
Here's what your code might look like:
<script>
function myCallback(jsonParam) {
alert("URL: " + jsonParam.href + " state: " + jsonParam.state);
}
</script>
<g:plusone size="medium" href="https://site.com" callback="myCallback"></g:plusone>
You can learn more about this attribute in the tag parameters section of the documentation.