Why Gmail Breaks Your Email CSS (and How to Catch It Before Your Users Do)

You spent an afternoon building a beautiful transactional email. The header gradient is perfect. The buttons have rounded corners and a subtle shadow. The layout uses flexbox so everything centres nicely. You send a test to yourself, open it in Gmail, and half the design is gone. This is not a bug. This is email in 2026. Email rendering is stuck somewhere around 2004. Gmail runs every incoming email through a custom HTML sanitizer that aggressively strips CSS it considers unsafe or unnecessary. Outlook desktop (2016 through 2021) goes further - it literally uses Microsoft Word as its HTML rendering engine. Word. The word processor. For rendering HTML. The result is that the CSS you write and the CSS your recipients see are often two completely different things. And unlike web development, there are no DevTools, no error messages, and no way to know what broke until someone screenshots it and sends it to you. Let's fix that. What Gmail Actually Strips Gmail's HTML sanitizer is not docum