Collapse a headline?

Yes — it’s absolutely possible, even with .shtml.

.shtml only affects server-side includes (SSI); on the frontend it behaves like normal HTML, so JavaScript accordion/collapse will work fine.

Simple solution (no libraries needed)

Add this where your content is:

<button class="accordion">Section Title</button>
<div class="panel">
<p>Your hidden content here</p>
</div>

Then before </body>:

<script>
document.querySelectorAll(".accordion").forEach(btn => {
btn.addEventListener("click", () => {
btn.classList.toggle("active");
let panel = btn.nextElementSibling;
panel.style.display = panel.style.display === "block" ? "none" : "block";
});
});
</script>

And basic CSS:

<style>
.panel { display: none; }
</style>

Notes:

• Placement matters → HTML where content is, JS before </body>
• Works in Dreamweaver and Windows 10 without issues
• If your YouTube example worked in .html, it will work the same in .shtml

👉 Your idea of loading .htm inside .shtml is correct (SSI #include), but not required for this feature.

I'm satified now with https://www.bassett.net/pages/AccordianSample.html

So the remaining question is why isn't  https://www.bassett.net/pages/david2.shtml  full width of the white area? Hopefully, I figure it out while waiting for a response, but... just in case.

Hmm, I figured out the include, I knew I'd used it before - it's in the sidebar. So, since posting original post, I've now included https://www.bassett.net/pages/AccordianSample.html

When pulling that page up in browser, the content uses the whole width. But when including it in https://www.bassett.net/pages/david2.shtml, it only uses about 25% of the available width. Why is that?

While waiting for an answer, I'll experiment on adding an image like a plus sign.