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:

• HTML syntax & structure: Validates HTML markup for unclosed tags, tag mismatches, unclosed comments, duplicate element IDs, and missing <!DOCTYPE html> declarations.
• 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.
• External resources & domain analysis: Groups all embedded remote images, fonts, and assets by domain. Warns if assets are hosted on local domains (localhost, .ddev.site, .local), staging/dev subdomains, non-standard development ports, or over insecure HTTP.
• Link & URL verification: Flags malformed URLs, relative paths without base href, broken in-document anchors (#anchor), unencoded spaces, unresolved template tags, unsupported protocols (file://, ftp://), placeholder links (href="#"), empty links, and localhost 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:

• Standard (Raw): Displays email HTML directly with all external images loaded.
• Outlook Desktop (Windows Word Engine): Strips <style> blocks, removes flexbox and grid layouts, clears modern CSS properties (box-shadow, border-radius, animations), blocks remote images by default with an Outlook information bar, and simulates Word table typography.
• Gmail Webmail: Strips position fixed and absolute styles, clears custom data attributes, blocks remote images by default with a Gmail notice bar, and wraps styles in Gmail web container constraints.
• Apple Mail: Renders with modern CSS3 and HTML5 capabilities and simulates privacy remote content blocking.
• External Images Toggle: Toggle between blocked remote images and loaded assets with a single click inside the preview banner or toolbar.
• 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.

Plain Text & Automated Fallback Extraction

Sinkbox inspects both HTML and plain-text parts of incoming email messages:

• Original Plain Text: When a message includes a dedicated text/plain part, Sinkbox displays it directly in the Text only tab.
• Automated HTML Fallback: If an email contains only HTML content without a plain-text version, Sinkbox automatically generates a readable plain-text fallback by stripping tags and decoding entities. The interface clearly labels this tab as Text only (generated) and displays an informative banner so you know the email lacked an original plain-text body.