What is the behavioral layer?
The structure or content layer of a web page is the underlying HTML code of that page. Just as a house's frame creates a strong foundation upon which the rest of the house is built, a solid foundation of HTML creates a platform upon which a website can be created. (ThoughtCo.)
You can make your site interactive by including some dynamic HTML functions.
This is the behavioral layer.
To be prepared for a discussion of the third web page layers, read over and watch the following.
... a method of designing websites so that the process or user request is run on the originating server.
Read What is Javascript? in a very popular online javascript book.
Read only the text under the What is Javascript? heading.