Website not working on iMac after iOS 16.3.1 update

Do you not mean macOS 26.3.1? Also give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

If Safari doesn't work as expected on Mac - Apple Support 

Cache keeps a snapshot of the site the last time you visited it.  

Clearing the cache may solve your problem.

Clear The Cache:

• Unhide the Develop menu: go to Safari Settings... > Advanced > Show features for web developers.

• Click the Develop menu in Safari's menu bar.

• Select: Empty Caches