the problem is: you can send only one POST and this content must have all the datas
let me use an example... my largest page has 14 sections, a mix between wysiwyg, code and image gallerys.
every section has the option to save the changes in a single process.
In a lot of modules, i change the content on subpages of this module, so in news, OFA, topics, form settings, bakery etc.
to coming back from this subpage to the section overview for this page with the "general save-button", i've to leave this subpage without saving and lost my changes
another example: news module - i change 3 news. now, i have two possible way's - save every single news or save all news on the overview. At the moment, i open a special item, have some hidden infos like section-id, item_id, summary ~15 hidden or visible fields in the POST to save this item. To use a solution like yours, i've to save this changes for one single item somewhere in the cache, local storage etc, to use it later in the "general save process".
These is only the news module...
Next section type is wysiwyg, in the section list on position 1, 3, 4, 8 , 9. Every section has ~ 10 fields to save - i dont know it exactly, is only an example...
Save this also in my cache or locale storage, some little changes here, some little changes there and every time you get more and more datas in your storage - and the risk, to lost something - in worst case, the changes from the last two hours or more, if the session is not ending...
it is possible, of course, but it make's no sense for me
you see only some small changes on the section overview page, like a couple of wysiwygs
a better solution, in my eyes, a intern list - if i set the focus to a special module (maybe to edit wysiwyg), i add this module into a list and if i save this content, i remove this entry. And before i go to a new page, the cms check this list - is it empty, go to the new page, is it not empty, write a warning and marked this section(s) with a light red color.
i saw solutions like this in other systems, but mostly for a special item (for example: if i write a answer in a forum and leave the page without sending)
other solution: if i set the focus to a special modul and change there something (like a wysiwyg section), the save-button get a new color, so that you know, i have to save this - but the problem is: you have a reload after the save-process and lost possible other changes, but it is a reminder - dont forget to save...