Google's +1 Button: How do they do it?

Exploring Google's +1 Button, I found two things odd about the code they supply:

<script type="text/javascript" src="https://apis.google.com/js/plusone.js">
  {lang: 'en-GB'}
</script>

<g:plusone size="tall" href="http://www.google.com"></g:plusone>

So I have two questions:
First:       How is Google able to use the text between the script tags?
Second:  Is the syntax <g:plusone ... HTML valid? What's this called?