Outlook email signature checker
Classic Outlook remains a special compatibility case because Microsoft documents that it uses a Word-based HTML processing engine with limited support for standard CSS behavior. New Outlook and Outlook on the web behave differently, so SigLint labels Classic Outlook findings explicitly instead of treating “Outlook” as one renderer.
What the checker looks for
The current Outlook-focused rules include compatibility heuristics for flexbox, CSS Grid and positioned layouts, plus Microsoft-backed checks for margin/spacing behavior, CSS background images and specific line-height patterns.
A heuristic finding means “this deserves a safer implementation or real-client test”; it does not mean Microsoft has published a rule naming that exact CSS feature.
Primary Microsoft evidence
Microsoft’s current email-rendering troubleshooting documentation states that Classic Outlook uses a Word-based HTML processing engine. It also documents specific limitations around margins/padding, CSS background images, numeric line-height values and line-height on span elements.
Read the Microsoft Learn source →
Recommended workflow
Use SigLint to locate risky constructs and understand the remediation direction, then re-run the checker after editing. A static linter is not a screenshot of Outlook, so important signatures should still be tested in the actual Outlook versions your audience uses.