1. Open the /templates-hidden/include.html file with a text editor
  2. Add the line:
<span data-lift="xform" data-css="footer *">Stuff for the footer</span>
  1. Save the file and publish your site

The /templates-hidden/include.html file is included in every page via the default.html and post.html template files.

The data-lift="xform" attribute says that this is a page transformation rule.

The data-css="footer *" attribute says, "find all the <footer> elements and substitute this element into the body of the found elements".