Email HTML Checker
Check email HTML against what Outlook, Gmail and Apple Mail actually support: unsupported CSS, Gmail clipping, missing alt text and dark mode problems.
Check email HTML against what Outlook, Gmail and Apple Mail actually support, before you send to a list.
Client compatibility
Will break
0
Risky
0
Fine
0
Nothing here will break in a major client.
Email HTML is not web HTML. Outlook on Windows still renders with Word's engine, which has no flexbox, no grid and no positioning, so layout has to be nested tables and inline styles. Every rule here is a documented failure rather than a preference.
Generating...About the email html checker
Email HTML is not web HTML, and that is the whole difficulty. Outlook on Windows still renders with Microsoft Word's engine, which has no flexbox, no grid and no positioning. Gmail strips anything it does not recognise. Nothing you learned building websites transfers cleanly.
The tools that test this properly cost around a hundred dollars a month, because they render your mail in real clients on real machines. That part genuinely is expensive. But most email failures are not subtle rendering differences: they are documented, predictable rules being broken, and those can be checked for nothing.
Every rule here is a reproducible failure in a named client rather than a matter of taste.
How to use it
- 1Paste your email HTML.
- 2Fix the errors first, since those break in a major client.
- 3Check the warnings, which are client specific risks.
- 4Send a real test to yourself before sending to a list.
Questions
Does this replace Litmus or Email on Acid?
No. Those render your mail in real clients and show you screenshots, which is worth paying for before a large send. This catches the rule breaking that causes most failures, for free, and it catches it while you are still writing rather than after.
Why does Outlook break everything?
Outlook on Windows renders email with Word's engine rather than a browser engine. It has no flexbox, no grid, no positioning and unreliable padding on anything that is not a table cell. That is why email layout is still nested tables in 2026.
Why does Gmail clip my email?
Gmail cuts anything over 102 KB and hides the rest behind a link. Everything below the cut, including your tracking pixel and unsubscribe footer, stops being seen, which quietly breaks your open rate.
Why does alt text matter so much in email?
Most clients block images by default, so on first open the reader sees your alt text and nothing else. An email built entirely from images with no alt arrives as a blank rectangle.
What breaks in dark mode?
Apple Mail and Outlook invert colours automatically. A dark logo on a white background becomes a dark logo on a dark background, because the background inverts and the image does not. Declaring color-scheme is what stops the automatic inversion.