What is rendering testing and which tools do it?
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You send a campaign that looks perfect in Gmail. Your designer signed off on it, the internal test send looked great, and you hit go. Someone on your team then checks it in Outlook and the layout has completely collapsed. Rendering testing is what you do before that moment happens.
Rendering testing means previewing your email across a set of actual email clients before you send. Because email clients use wildly different rendering engines, what displays correctly in one can break completely in another. Litmus and Email on Acid are the two main tools: both take your HTML, run it through dozens of real client environments, and return screenshots of how it looks in each one. You'd check the clients your audience actually uses, which you can pull from your ESP's client breakdown report.
What you're looking for: broken layouts, missing fonts, images that aren't loading, buttons that aren't tappable on mobile, and dark-mode color inversions that make text unreadable. Outlook is historically the most problematic client because it renders HTML using Microsoft Word's engine rather than a browser engine, so CSS that works everywhere else often doesn't work there. Most rendering failures you'll encounter in production trace back to client fragmentation in the Outlook family.
A practical workflow: identify your top 10 email clients from your analytics, run a rendering preview in Litmus or Email on Acid, fix critical issues in your template, and retest before every send. If your list is heavily concentrated in one client, prioritize that one and treat others as secondary checks. Start by pulling your client breakdown from your ESP's analytics so you know which environments actually matter for your audience.
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