I wish they took the use-case of resizable panels into account. Even if the implementation is fully user-land, a styleable separator that can also receive events would be so useful.
Yeah they were ugly and looked like they came straight out of the Netscape 2.0 era (because they did) but that’s nothing CSS couldn’t have fixed.
At one point people decided that tables are lame.
Iframes on the other hand posed a security issue (cross site scripting) - you have two pages living inside the same window, with possible access and trivial attack vectors like creating a page with one iframe being invisible (1x1px for instance).
But that's just an exception, frontend is really a fashion driven developement and there's often no rhyme or reason to why the mob decides to chose one way over another.
A native splitter is one that is drawn by the browser itself, or even the OS. The only way to get that on the web is with iframes. Otherwise you have to resort to Javascript and manually handling mouse move events as in the example above.
Why do you want that? Same reasons we have `<input>` instead of having every website reimplement text boxes, sliders, checkboxes, etc. with canvas and mouse events.
Sorry I couldn't find an example of an actual iframes website because obviously nobody uses iframes any more.
Your example has `noresize` set, but I found one that doesn't: https://www.yourhtmlsource.com/examples/frameset1.html