Tier A — first-party
Current documentation published by the email client or platform vendor, such as Microsoft or Google. These are preferred for high-impact claims.
What SigLint checks — and what it does not
SigLint performs deterministic static analysis of HTML email-signature markup. It looks for specific constructs that have documented or well-understood compatibility risk, locates them in the source, and explains a safer implementation direction.
SigLint is not a rendering farm. A passing result is not a guarantee that every Outlook, Gmail, Apple Mail or mobile version will display a signature identically. Important signatures should still be tested in the real clients used by the intended audience.
Evidence tiers
Tier B — reproducible compatibility evidence
Independent client testing or datasets with clear methodology and provenance. These can support rules after the behavior is reviewed and reproduced.
Tier C — observed/community evidence
Useful signals that need additional validation before SigLint treats them as strong compatibility claims.
Heuristic — conservative lint
Deterministic patterns such as scripts, external stylesheets or relative asset URLs where SigLint can give practical guidance without pretending the rule is a vendor guarantee.
Severity
High means the pattern can make important signature content or layout unreliable enough that it should be reviewed before use. Medium means compatibility can vary and the pattern deserves deliberate testing or a safer fallback. Low covers quality, accessibility or layout-stability improvements that are unlikely to destroy the whole signature.
Severity is not a probability and is intentionally not converted into a 0–100 compatibility score.
Why Outlook gets special treatment
Microsoft documents that Classic Outlook uses a Word-based HTML processing engine with limited support for standard CSS behavior. New Outlook and Outlook on the web have different rendering behavior. SigLint therefore labels Classic Outlook findings explicitly instead of presenting one universal “Outlook” result.
Current Microsoft evidence
Microsoft Learn — troubleshoot email rendering
Current source-backed rules include Classic Outlook margin/spacing behavior, CSS background images, unitless line-height and line-height applied to span elements.
Gmail rules
Gmail rules are kept narrow. Google currently documents a 10,000-character signature limit, including signature images toward that limit. SigLint checks the HTML length against that documented constraint.
Current Google evidence
Source locations
Where a rule is regex-backed, SigLint reports the first detected line and column, shows a source snippet and provides a “View in HTML” action. This is diagnostic assistance, not a full HTML parser or IDE. Multiple-occurrence reporting is planned so repeated problems are not hidden behind the first match.
Rule maintenance
Source-backed rules carry a verification date. Material changes to detector behavior, client scope, severity or evidence should be recorded in the changelog. The public Rules Library is the current coverage contract.
Privacy
The core checker runs locally in the browser. The current release does not send pasted or uploaded signature HTML to a SigLint application backend. Standard hosting infrastructure can still receive normal web request metadata when pages and assets are loaded.
What a clean result means
“No current rules matched” means exactly that: the current rule corpus did not detect a known pattern. It does not certify the signature, prove accessibility conformance or replace testing in real email clients.