HTML emails cheatsheet

Property	Where
position	(Outlook, Gmail, Yahoo)
display	(Outlook, Gmail)
float	(Outlook)
height	(Outlook)
width in p/div	(Outlook)
padding in p/div	(Outlook)
background	(Outlook, Gmail)
min-width	(Outlook)
max-width	(Outlook)
opacity	(Outlook, Gmail, Yahoo)
box-shadow	(Outlook, Gmail, Yahoo)
rgba()	(Outlook)
data-uri	(all webmail)

E[attr]	(Outlook, Gmail)
E:nth-child(n)	(Outlook, Gmail)
::before and ::after	(Outlook, Yahoo, Gmail)
E F	(Gmail)
E + F, E > F etc	(Outlook, Gmail)

Inline your CSS as much as possible.

<table cellpadding="0" cellspacing="0" role="presentation">
  <tr>
    <td width="auto"></td>
    <td width="600" background="#ffffff">
      ···
    </td>
    <td width="auto"></td>
  </tr>
</table>

<style>
@media only screen and (max-device-width: 480px)
</style>

<style> is supported in the head and body by everything except Gmail. Only use them for responsive styles.