p51Labs came up with the Simply-Buttons v2. Buttons auto resize itself to fit text. There are 3 states: Inactive, Active, and Hover. They look and behave the same way in every browser and operating system. However, the caveats are the outlines on the buttons, the text is selectable. And These buttons are not mobile friendly.

With Simply-Buttons Javascript component provided, you can solve the caveats above as well. You can easily modify the buttons look and feel. You can do this by creating your own stylesheet. If you like this, you might be interested in Simple Round CSS Buttons or Colorful and Scalable Buttons or Beautiful Scalable CSS Buttons as well.

Requirements: -
Demo: http://www.p51labs.com/simply-buttons-v2/
License: License Free