Java applets and ASP.Net did have a superficial answer to this, as well as Flash, but they varied in terms of their ability to actually function as raw web interfaces using the URL to navigate between sections.

Being able to reliably and programmatically interact with client-side storage and the url, as well as improvements in DOM apis and commodification of hardware with more ram and faster faster CPUs, among many others factors, seem to have contributed.

ExtJS came about in the late 00s, and it was possible to build SPAs with it, overlay/draggable windows, and had powerful grid/form components.

https://extjs.cachefly.net/ext-3.4.0/examples/ unfortunately most of the data examples don’t work, but I believe everyone browsing the web back then remembers this blue tinted interface.

oh man now THAT is a real blast from the past. oh man, pretty classic "where did all the time go?"... "oh yeah i forgot that i spent like 100s of hours battling with sencha back in the day". selective memory isn't always a bad thing, i guess.

Java applets are entirely distinct from SPAs, at least as that term is used in the webdev community.

Hm, alrighty then. Seems like an unnecessary distinction to me in that it deeply constrains what SPAs can be, but I'm not a webdev.

The rule is either "uses html to render" or an even looser "renders in the browser". That doesn't seem like a deep constraint to me. You exclude a couple very popular but historical plugins where the browser set up a rectangle and handed it off to an external piece of code, and pretty much nothing else.

Those apps came from an era when these terms weren't even coined yet.

I think of java applets as being more akin to wasm; SPAs were/are a collection of html that recreate the experience of a "plain old" html web site.