Most companies and email providers have a web interface to allow access to email. People, who read email, generally use many
different web browser to access them. At home you might use Mozilla Firefox to view your mail, at work perhaps Microsoft
Internet Explorer, and on the go Apple's Mobile Sarfari browser on your iPhone.
As you know every browser and every version of every browser renders HTML and CSS differently. They all have different HTML and
CSS support therefore, making it very important when creating an email, newsletter, or website that you design renders and works correctly
in all browsers.
If you read all the different references listed above, you're head is probably spinning right now. It's a lot to know and absorb. Don't let it scare you off, just keep in mind
when you design HTML emails using the most common HTML Tags and CSS Styles, chances are it will render correctly in most browsers. Thats why we recommend the
KISS Principle (Keep It Simple Stupid) when designing HTML email for cross browser support. For safe HTML Tags and CSS styles that renders the same in most browsers, reference the chart
located on the bottom of this resource: Formatting HTML emails for Email clients.
Source: Unlock The Inbox