Hi Peter,
Thanks for emailing. requestAutocomplete is really focused on entering information that's reused across many websites, rather than something like site-specific ids and passwords, which are only used on a single website.
In Chrome, the Autofill feature includes as a subset the functionality that's referred to as "autocomplete" in several other browsers. This is intended to support the use cases that you've described, though it has some limitations: It won't remember data for fields marked autocomplete="off", and it won't save numbers that "look like credit card numbers", which might be triggering for your 14-digit library card [1]. If you see any bugs beyond that, please file them on the Chromium bug tracker:
http://crbug.com
Cheers,
~ilya