how can I force automatic filling to be enabled on webpages?
Inherently there is no option to "auto-enable" completion even if a web page has disabled it. The web page disables it using the property autocomplete=off in an element. There is a chrome extension: Auto-Complete = On
You can use that to turn on auto-complete for the elements that are disabled.
If you do not want to install an extension:
You can right click on the form, select Inspect Element and edit the auto-complete attribute of the <form> element directly: change autocomplete="off" to autocomplete="".