This article describes what the Autofill feature within your browser is and how you can use it to save your Yahoo! login information. 

Browsers can detect your credentials when you sign in to certain sites. You can enable the Autofill feature so that your password will automatically fill in on future visits. Once enabled and your password is remembered, you'll only need to click Sign in to access your content. 

Note: If you've signed out or cleared your cache and cookies, you may have to enter your Yahoo! ID the next time you visit a Yahoo! sign-in screen, but your password should auto-populate once the Yahoo! ID is entered.