This technique relies on adding thick borders to the link itself to build the button's CTA. Using borders is universally understood by email clients, but limit button appearance solid colors.
<table width="100%" border="0" cellspacing="0" cellpadding="0"> <tr> <td> <table border="0" cellspacing="0" cellpadding="0"> <tr> <td bgcolor="#0095FF" style="padding: 12px 18px 12px 18px; -webkit-border-radius:3px; border-radius:3px" align="center"><a href="http://www.stackoverflow.com" target="_blank" style="font-size: 14px; font-family: sans-serif; font-weight: normal; color: #ffffff; text-decoration: none; display: inline-block;">Stack Overflow</a></td> </tr> </table> </td> </tr> </table>
The link tag is set to be a block-level element and that borders are used to provide the padding, so the entire button is hoverable + clickable, even in older desktop clients.
The drawback of border-based-buttons is that Outlook will reduce the size of the borders by a small amount and, as mentioned above, background images are not supported.