Vanilla Javascript snippet to show an header

I need a code snippet to add to every site where I need this header. The injected header will be the same on all websites and will not have any external dependencies. No jquery, no bootstrap. All the classes should be unique to this snippet so it will not break the page where it is added.
Here an example that uses jQuery and Bootstrap: http://preview.monforte.it/gfl-oldtoolkit/header/
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