When using an ordinary <select> in Firefox and Chromium (and probably other browsers), you can type letters at it and it will jump to the first item in the list that matches those letters. See for example https://en.wikipedia.org/wiki/Special:ApiSandbox, open the "Action" dropdown and type "feedw": the selection will first jump to "fancycaptchareload", then "featuredfeed", then "feedcontributions", then "feedwatchlist" as the letters are typed.

To be a useful replacement for <select>, OO.ui.DropdownWidget should have the same behavior.