The first thing that happens in this example is that we build an output tree—a structural skeleton for the file we are creating. If we look at this diagram we can see that the stylesheet contains an entire miniature HTML document within this template.
When we run this particular stylesheet, we say, in effect I have a paragraph here. What do I do with a paragraph? The stylesheet answers, When I have a paragraph, I create an output tree like this!
Notice that the output tree can include some predefined content (like the
<head> and <title> in the example).
But where does the content of our input document (i.e. Hello, World!) show up in our output document?
