CSS, or Cascading Style Sheets, is the best way to style HTML. But it's important to know how different email clients hande CSS.  Some do not accept CSS styling at all, while others accept most styles.

While many desktop and web email clients will show "style" elements in the HTML "head", for example, some do not, including the ever more popular Gmail, which strips out most CSS to a bare bones appearance. External style sheets are often screened out, so they aren't a good method for styling emails. "Text" and "font" properties are generally safe to use, but even those will not show up in the stray email client.

Make sure that chosen styles will not adversely affect the email if they are absent. Test email in different clients prior to sending. For more information on specific CSS properties in email clients, check out these great charts and links:

• http://www.campaignmonitor.com/css/
• http://onenw.org/toolkit/guidelines-for-css-and-email-newsletters/
• http://www.alistapart.com/articles/cssemail/