[founderling]

    the WC equivalent of <include> is the <script> tag

I beg to differ. My <include src="footer.html"> would insert the html in the footer.html file. While <script src="components.js"></script> will execute javascript. Then the javascript has to somehow prepare magic so that <hn-footer> will be replaced by some html.

The link to the library you gave does not help here. Because I am thinking about browser technology here. What has to go into components.js (without using a library) to make <hn-footer> be replaced with the html in footer.html?