Use synthetic final rendered HTML and one independently expected copy, displayed-value or CTA-destination rule. The browser preview needs no signup. It checks supplied HTML; it does not run your templates, validate tokens or payments, or test delivery or inbox appearance.
What rule did you attempt and what result did you observe? Including setup, false positives and rule maintenance, does it add value beyond your existing tests? If useful for recurring business work, what would make these current checks worth paying for?
Update: the preview now shows the text a failed rule expected and what it actually found. In the synthetic EUR receipt, the result points to #total and shows €24.00 vs €29.00. The USD and discount variants can be checked separately on the scenario page.
One useful habit, even if you already use a test runner: take the expected total from a known fixture or business rule, then render the email and compare the selected text. If the same formatter generates both values, a formatting bug can pass both sides.
If you are changing a receipt or invitation right now, I would be interested in one rule from that change: what do you check today, and would the free preview linked above replace any step? A dummy example or a description is enough.
