GOOD

Buttons

Primary

<button>Submit</button>
Button

Take Action

<p class="action"><a href="…">Take Action</a></p>
Link

Secondary

<form action="…" method="post" class="secondary">
  <button type="submit">Flag this post</button>
</form>
Button

Skip this

<p class="action secondary"><a href="…">Skip this</a></p>
Link

Edit this post

<p class="action secondary edit"><a href="">Edit this post</a></p>
Link

Warning

<form action="…" method="post" class="warning">
  <button type="submit">Delete</button>
</form>

Facebook

<p class="action facebook"><a href="…">Find your friends</a></p>