<!-- local file: if test.html is in the same directory -->
<iframe src="test.html" width="1000" height="1000">
<!-- local file: if test.html is in the parent directory -->
<iframe src="../test.html" width="1000" height="1000">
<!-- online iframe (youtube as an example) -->
<iframe src="https://www.youtube.com/embed/s4BibernJxU" width="1000" height="1000">