I am trying to create a split-button in my website (ASPNET WebForms). So far the only split-button implementation I like is the one from Bootstrap. I am not using any other feat
You go to the Customizer on getbootstrap.com http://getbootstrap.com/customize/
Toggle the stuff you need:
Then you download it.
You open it up. Look at the base styles and the global box-sizing:border-box.
Either you change all your CSS to work with box-sizing: border-box (google it), or you put that on the :before, :after, and element on all the bootstrap css that you need for forms, buttons, and the dropdown (they work together).