The example on that site of the gradient button with "Preview" text is broken. The text isn't centered because the container is set to display: flex but no flex-grow set to expand the content box to fill the parent. It has text-center set, but because the box is collapsing that's not working as expected. Really, it needs Tailwind's justify-items-center applied to it.

I'd worry about using a framework whose primary example is wrong.