This workaround doesn't seem to work anymore on XWiki 14.10.x...
I tried with both async=false and async=true in the URL and in the form and I cannot seem to be able to get the save to not be async...

This is a bit of a mess as validation remains a feature of the core save action: if validation is asked for (as decribed in the tutorial https://www.xwiki.org/xwiki/bin/view/Documentation/DevGuide/Tutorials/Creating%20a%20form%20with%20validation%20and%20tooltips ) and invalid data is submitted, then the page is not actually saved but errors are not displayed either, it behaves as if it did save.

You will notice that the form fields are readonly and a notification saying 'Saving ...0%' is displayed at the bottom of the page, then nothing else happens (see the screenshot)

on version 14.10 the "Saving..." actually goes through and the page is reloaded in view mode, as if a proper save took place (but it's not the case).

This may also be related to XWIKI-20483 : the http answer that the save submit receives is the actual edit mode of the page with errors, but it ignores it and announces success.