Most of the heavy lifting of modern web applications is still on "userland" JavaScript frameworks instead of the browser, because lots of dynamic interactions can't really be modelled with HTML alone. For example, consider creating a dropdown menu to select a contact, displayed with an avatar, name, and email as a subtitle. Doing that right—so it is accessible to screenreaders, has dynamic completions fetched from a server, adjusts to different output devices—is not only quite complex on its own, but also requires reinvention of several things, like an input field with a custom select box. Support for modal dialogs just became widely available two years ago. Two way data bindings between dynamic state and HTML elements still requires complex Javascript code. Drag and drop is a pain. Input elements haven't really caught up to the last twenty years.
And while HTML does provide capabilities to register custom elements, they don't really catch on for a reason.
I will say HTML is flexible enough to be used for just about anything, but it certainly is better suited for text documents than web applications.
The OpenUI folks have been doing the angel's work of upstreaming various core components. And doing a great job. Popover & dialog are both coming along nicely. https://open-ui.org/
For a while it was all left to appdevs to build a UI toolkit out of basic HTML but OpenUI really has been driving the standard forwards.
Why would you need a custom selectbox in the example you provided? Because you could use the menu element with the popup API and AJAX if I understand your example correctly.
On a sidenote, do you know about HTMX? If so, what do you think about it?
The popover API is pretty much brand new. Sure, that would probably work. That misses my point, however; lots of basic building blocks of modern UIs users expect are missing from browsers today (or on the popover case, yesterday) and need to be added manually.
HTMX is the same story—it's what HTML could be, but isn't, unless monkey-patched in userland.
Yes it's still and always evolving and we need less and less of the heavy JS frameworks to produce interfaces. HTMX is way lighter than React, for instance. And from what I know, it's enough for many, many app use cases.
I will say HTML is flexible enough to be used for just about anything, but it certainly is better suited for text documents than web applications.