Is Htmx just the long overdue successor to DHTML that was all the hype in, say, 1997?
Dynamic HTML was a marketing term used by Netscape and Microsoft to refer to the combination of HTML, CSS, and JavaScript. Also under that umbrella term you have the introduction of the div tag for IE or the layer tag for NS4 (also NS4 included a JS version of CSS)
It was “dynamic” because you could do effects like hover menus using JS snippets or the first iterations of CSS (ohh the memories of my first Geocities pages). But, XHR APIs didn’t existed yet.
Edit: it seems that the marketing term was used mainly by Microsoft for IE.