Not necessarily. It depends on what you need. For example suppose you were implementing an news reader with two panes. Left pane is a list of titles and previews, right pane is a full article for the selected item in the left pane.

You can make the left pane a list of links to dedicated pages, and then add an htmx attribute to indicate that when the user has JavaScript to instead fetch a html snippet with just full article and swap it into the right pane.

If you had a reasonably bounded set of articles you could statically generate all the /article/id.html and the /article/id/snippet.html files, though a backend probably makes sense.

In that case you need to have everything rendered server side and perhaps get creative with the usage of forms firing off events that can be intercepted by the server and then return a new page.

Certain things like Nav menu drop downs and folding text can’t be done with native CSS and HTML.