I am able to divide the whole page into 4 regions and they are:-
[header] (on the very top), [nav] (right below the header), [section] (on the bottom left) and [main] (on the bottom right).
In the [nav] portion, I have a list of hyperlinked items (like “item-A”, “item-B”, etc).
The task is to:-
When “item-A” is selected, the corresponding sub-list containing “A1”, “A2”, “A3” etc should be displayed on the [section] part. Similarly, if “item-B” is selected, the sub-list “B1”, “B2” should also be displayed on the [section] part (overwriting the previous content, of course).
“B2” again is hyperlinked to the file “B.html” which, when called, should be executed on the [main] portion.
How can I do all of the above?
P.S. I can finish the above if I use [frame] and [frameset]. Unfortunately, these become obsolete in HTML5.
The widths and heights of each region can be defined inside the CSS setup.

Stack Blitz code: https://stackblitz.com/edit/web-platform-uwbmw4?devtoolsheight=33&file=index.html
Because it needs a1,a2,b1,b2 HTML files and stuffs it does not work here(below)(other parts work other than iframe stuff) please try above stackblitz for testing