Flexbox layout detected
HighFlags display:flex and inline-flex in signature HTML. Guide →
Transparent rule coverage
SigLint Rules Library
SigLint is a deterministic linter. This page makes its current compatibility coverage visible instead of hiding it behind a score. Release 0.1 currently includes 22 checks across Outlook Classic, Gmail, general email-client behavior, images, layout stability and accessibility.
How to read the library
Tier A means the rule is directly backed by current first-party client/vendor documentation. Other checks are conservative deterministic compatibility heuristics and are deliberately described as such. A finding is evidence of risk, not a guarantee that a client will render incorrectly.
Outlook Classic
Checks aimed at the Word-based HTML processing behavior used by Classic Outlook.
CSS Grid layout detected
HighFlags Grid-based critical layout and recommends table-based structure for compatibility-sensitive signatures.
Margin-based spacing detected
MediumMicrosoft documents inconsistent margin and padding behavior in Classic Outlook custom HTML.
Absolute/fixed positioning detected
HighFlags positioned layout that should not be relied on for signature structure.
CSS background image detected
MediumMicrosoft documents that Classic Outlook does not natively support CSS background images on sections and columns.
Unitless line-height detected
MediumFlags numeric values such as line-height:1.5; Microsoft recommends percentage values for Classic Outlook custom HTML.
Line-height applied to a span
MediumFlags line-height on span elements because Microsoft documents spacing artifacts in this pattern.
Gmail
Signature exceeds 10,000 characters
HighGoogle documents a 10,000-character Gmail signature limit. Guide →
General email-client compatibility
CSS custom property detected
MediumFlags reliance on CSS variables for critical presentation.
Embedded style block detected
MediumRecommends inline critical styling for signatures.
External stylesheet detected
HighFlags linked stylesheets that are not dependable in email messages.
SVG image detected
HighFlags SVG assets where broad signature compatibility matters. Guide →
Embedded data-URI image detected
HighFlags base64/data-URI images. Guide →
Script detected
HighJavaScript is not a dependable email-signature primitive.
Video/media element detected
HighFlags video, audio, iframe and canvas elements.
Web-font dependency detected
MediumRecommends a robust system-font fallback stack.
Very wide fixed layout detected
MediumFlags fixed widths likely to behave poorly on small screens.
Non-HTTPS image detected
MediumFlags insecure remote image URLs.
Relative image URL detected
HighRelative asset paths have no dependable page base after signature HTML leaves its original site. Guide →
Form control detected
HighFlags forms, inputs, buttons, selects and textareas in signature HTML.
Accessibility and layout stability
Image without alt text detected
LowPrompts for useful alternative text or an empty alt attribute for decorative images.
Image missing explicit dimensions
LowEncourages explicit image sizing to make intended signature dimensions clearer and reduce layout surprises.
Primary evidence currently used
Microsoft Learn — email rendering troubleshooting
Google Gmail Help — create a Gmail signature
SigLint records verification dates for source-backed rules and will update this library when rule behavior, evidence or severity changes.