Fields that autocomplete should be representable with a combobox, instead of just a standard autocompleting input - the difference is that, in a combobox, the user can also see all the available values through a dropdown. A combobox should probably be the default option, and maybe the only option, for autocompletion fields. The best Javascript library to handle this might be Ext; here's a demo for their combobox widget:
http://extjs.com/deploy/ext/examples/form/combos.html
Version: unspecified
Severity: enhancement