Forgot password isn't enough, what if I forgot my login ID? But then it circles back to my point--what _is_ an identity? Would the button say 'forgot email'?

Forgot password isn't enough, what if I forgot my login ID? But then it circles back to my point--what _is_ an identity? Would the button say 'forgot email'? But not every site uses email as principle ID... It gets even more complicated when I have multiple accounts. Who lets me pick what account I want to use? Is the site responsible for that, or does the browser now need a concept of storing multiple IDs (whatever they might be) and managing them? It gets extremely complicated extremely fast.