IMPORTANT UPDATE !
I'm adding this update in hopes that it might help others who have experienced the same issue.
The two most likely solutions were the "Clear History..." option within the main "Safari" top menu, and the "Empty Caches" option, halfway down the "Develop" top menu (if you happen to have loaded it). Neither options solves my issue.
The "Clear History..." option (in the "Safari" menu) does indeed "clear the history". It's really just an exact replication of the "Clear History..." option, in the "History" menu. (I'm not really sure why Apple duplicated an action in two different menus, but they did.)
I still haven't figured out what the "Empty Caches" option in the "Develop" menu is supposed to do... presumably it does clear some cache somewhere! But it does not seem to have any effect on the ability to refresh a page with just-changed coding, in order to display said changes. Unless someone happened to be "tweaking" styling code and/or content in a local development (like I was, in a local MAMP Pro installation), most would likely never notice whether it was [or was not] working as expected.
I've closely watched my personal "cache" directory (sorted in descending order, by date). I see a range of caches moving to the top of the sorted list, but nothing that would appear to be a likely option to be a solution, should I delete it from my user cache... so I didn't.
One thing that I did determine is that it takes far more effort to view changes in styles than changes in content. I suspect that changes only affecting the way things are displayed, are not nearly as likely to cause a clearing of the cache than changes affecting what is displayed.
It's unlikely that many will experience the same issues, unless they happen to be doing the same tweaks, while developing code. But, on the odd chance it might help someone else, I thought it worth the effort to make this update.
Thanks to those who made suggestions along the way!