Here's some examples of the buttons styles used in the payments-ui.
<h2>Buttons</h2>
<button type="submit">Button</button>
<button disabled type="submit">Button (disabled)</button>
<h2>Link Buttons</h2> <a class="button" href="#">Button Link</a> <a class="button disabled" href="#">Button Link (disabled)</a>
These are the spinner buttons.
<h2>Buttons With Spinners</h2>
<button>Standard Button</button>
<button class="spinner">Button with Spinner</button>
<button disabled class="spinner">Button (disabled)</button>