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HTML email compatibility checker

Scans pasted HTML email code against a bundled client support matrix for Gmail, Outlook and Word, accessibility and the Gmail clipping size, reporting problems, warnings and notes with line numbers.
Email HTML source

This is a static scan of the markup against a bundled support matrix, not a rendering: Gmail's published CSS list, Microsoft's Word engine behavior for classic Outlook, and Gmail's 102 KB clip size. It cannot catch spacing quirks, image cropping, or dark-mode inversion, so send yourself a test email before a real campaign. Content inside Outlook conditional comments is scanned too, because Outlook renders it.
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Canonical HTML: https://nutter.tools/web/html-email-compatibility-checker
Markdown version: https://nutter.tools/web/html-email-compatibility-checker.md
Plain-text version: https://nutter.tools/web/html-email-compatibility-checker.txt
Agent index: https://nutter.tools/llms.txt