Looksgoodmail
Email HTML ships to a thirty-year-old web with three rendering engines and no spec. Outlook strips classes. Gmail clips at 102 KB. iOS adds blue underlines to phone numbers. Most QA today is pay-per-test (Litmus, Email on Acid) or eyeballing screenshots and hoping.
Looksgoodmail does the deterministic version: rule-based compatibility checks against 26 clients across desktop, web, and mobile, plus safe automatic fixes for common issues like CTA styling and image dimensions. Pass/fail with a transformation diff — no render farm, no waiting.
The rule set is small enough to read end-to-end, and the test corpus runs in CI. If a fix would change the visual meaning of an email, it's flagged rather than applied silently.