Email HTML Linter & Simulator

Sinkbox includes a built-in email HTML linter and simulated client rendering engine powered by Can I Email support data.

Compatibility & Lint Analysis

When viewing an email, the Compatibility & Lint tab automatically performs thorough checks on your HTML markup and CSS styling:

• Non-inlined CSS detection: Flags styles in <style> blocks that are not inlined onto elements, which Outlook Windows Word engine will ignore or strip.
• Flexbox & CSS Grid warnings: Detects modern CSS layout properties (display: flex, display: grid, gap) that lack support in Outlook desktop and older mobile clients.
• Accessibility & images: Checks for missing alt attributes, missing explicit width and height dimensions, and base64 data URIs.
• Link verification: Flags placeholder links (href="#"), empty links, and localhost or local development destination URLs.
• Advanced client rules: Identifies interactive form tags, embedded media, CSS variables, keyframe animations, background images without VML fallbacks, and payload sizes exceeding the 102 kB Gmail clipping threshold.

Client Simulation ("Fake rendering")

Test how your email markup looks under different email client constraints without needing real device test suites:

• Outlook Desktop (Windows Word Engine): Strips <style> blocks, removes flexbox and grid layouts, clears modern CSS properties (box-shadow, border-radius, animations), and simulates Word table typography.
• Gmail Webmail: Strips position fixed and absolute styles, clears custom data attributes, and wraps styles in Gmail web container constraints.
• Apple Mail: Renders with full modern CSS3 and HTML5 capabilities.
• Dark Mode Simulation: Simulates email client automated color inversion (light backgrounds inverted to dark shades, dark text to light shades) and activates @media (prefers-color-scheme: dark) styles.
• Responsive Viewport Switcher: Preview email layouts on Desktop (100%), Tablet (768px), and Mobile (375px) device viewports.