Usually cross-browser compatibility is the name used for testing websites, to see if a website is displaying correctly in different brands and/or versions of web browsers. Does your website look the same in Internet Explorer 9 as it does in Internet Explorer 11 or Firefox or Google Chrome or Safari or any other popular browser? Is the functionality the same in all these browsers? Does every script do what it did in the browser it was first developed on?

While this is all very important for websites, it is relatively easy to test. Just install all these browsers on your computer, and verify.
Ok, it is a bit more complex then that, but pretty close.

Not so for email clients. The concept is the same, but there are so many more email clients available, and to be cross-browser compatible with all these email clients, a lot of testing and comparing needs to be done.

Luckily there are some online services available that generate screenshots of various email clients, but these are expensive Paid Services with monthly subscriptions. You don't subscribe to these if you only send out or build the occasional email template.
This is where numbers start counting. We build templates daily, and test them daily. For us it is only logical to have accounts for these services, and we test all templates we make for correct cross-browser compatible behavior. Have us build your templates, and you can be assured of a correctly behaving cross-browser compatible HTML template.

We test all our templates through Litmus