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ToolApr 9, 20269 min read

HTML Email Checker

Michael Johnson — Full-stack engineer from Austin, Texas. Michael built the compatibility checker based on 10+ years of email debugging. Details the rules engine, scoring methodology, and how to interpret results.

Before you hit send, check your HTML email for compatibility issues. MailViewr's built-in checker scans your code against real email client rules and gives you an instant compatibility score. This is a key component of our ultimate guide to HTML email testing.

What Does an HTML Email Checker Do?

An HTML email checker scans your email's HTML and CSS code for properties and patterns that cause rendering problems in specific email clients. It's like a linter, but specifically built for email development.

MailViewr's checker tests against rules for:

  • Gmail — Chrome, Firefox, Safari, iOS, Android
  • Outlook — Desktop (Word engine), iOS, Android
  • Apple Mail — macOS and iOS

What Issues Does It Detect?

Common Issues Found

  • Unsupported CSS properties per client
  • Missing fallback fonts
  • Images without alt text
  • Missing role="presentation" on layout tables
  • External stylesheets (not supported)
  • CSS Grid / Flexbox usage
  • Missing DOCTYPE declaration

What You Get

  • Overall compatibility score (0-100%)
  • Error count by severity
  • Per-client breakdown
  • Specific line references
  • Actionable fix suggestions
  • Dark mode compatibility notes
  • Instant results (no waiting)

How to Use the Checker

  1. Paste your HTML into MailViewr's email preview tool (or use the visual builder)
  2. Select the email clients you want to check against
  3. The compatibility score updates automatically as you type
  4. Click on any issue to see the fix suggestion
  5. Preview the rendered result to verify your fixes
Tip: The checker runs in real time. As you edit your HTML, the score and issue count update instantly — no need to re-run anything.

Is This a Litmus or Email on Acid Alternative?

If you're looking for a free alternative to Litmus or Email on Acid for checking email compatibility, MailViewr covers the core use case: scan your HTML for client-specific issues and preview the rendering — all in the browser, completely free, no signup needed.

For teams that need screenshot-based testing across dozens of clients, paid tools offer that. But for checking CSS compatibility, spotting rendering issues, and previewing across the major clients — MailViewr has you covered.

Check Your Email Now

Free HTML Email Checker

Paste your HTML and get an instant compatibility score across Gmail, Outlook, and mobile. No signup required.

Check Email Free →