What is rendering testing and which tools do it?

Rendering testing captures how your email actually appears across different email clients, devices, and operating systems. Because each client interprets HTML and CSS differently-sometimes dramatically so-what looks perfect in your design tool may break in Outlook or display strangely on Android. Rendering tools automate the process of checking dozens of environments without manually testing each one.

The industry standard tools are Litmus and Email on Acid. Both services send your email through their testing infrastructure and return screenshots from 90+ client/device combinations: various versions of Outlook, Gmail's web and mobile interfaces, Apple Mail across iOS and macOS, Yahoo, AOL, and more. They also offer additional features like accessibility checking, link validation, and spam testing. Some ESPs include basic rendering previews built into their platforms.

Effective rendering testing requires knowing where your audience actually reads email. Your analytics should tell you the top 10 clients representing 80-90% of opens. Focus testing on those environments-exhaustive testing across obscure clients has diminishing returns. Rendering testing turns the unpredictable chaos of email client fragmentation into a manageable checklist; skipping it means hoping your code works everywhere when hope is not a strategy.