All of our tests are run through actual email clients. We strive to make sure that all renderings are accurate.
If you run an acid test and find a discrepancy (an inconsistency between our previews and the way your email actually renders in any of our supported clients), we want to hear about it! For that reason, we have a "Feedback" feature built into our Developer Tools (next to a screenshot, the "</>" icon). When using this feature, there is no need to submit your code or URL because we include your code in the support ticket.