The rendering linter for email
Quick Start · What It Catches · Why Emailens · Supported Clients · API Docs · The State of Email CSS
Your email looks perfect in Apple Mail. Gmail strips half the CSS. Outlook renders it in Word.
@emailens/engine analyzes your email against 255 CSS and HTML features across 21 email clients, scores compatibility, and shows you exactly what to fix — before you hit send.
Write it however you like. HTML, MJML, Maizzle and React Email all go in the front: the engine compiles the template with your own compiler and lints the HTML that actually gets sent, which is the only version your reader sees.
Our data says you need this: of those 255 features, only 6 are fully supported in every major email client. See The State of Email CSS.
emailens.dev — Try the hosted version. Paste HTML, get a full audit in seconds.
No install, no project setup, lint any email right now:
npx @emailens/cli lint email.htmlOr use the engine as a library:
npm install @emailens/engineimport { auditEmail } from "@emailens/engine";
// Flexbox + gap + box-shadow — all Outlook killers
const html = `<html lang="en">
<head><title>Weekly Update</title>
<style>
.card { border-radius: 8px; box-shadow: 0 2px 8px rgba(0,0,0,0.1); }
</style>
</head>
<body>
<div class="card" style="display: flex; gap: 16px;">
<div>Column A</div>
<div>Column B</div>
</div>
</body>
</html>`;
const report = auditEmail(html, { framework: "jsx" });
console.log(report.compatibility.scores["outlook-windows"]);
// { score: 30, errors: 3, warnings: 3, info: 1 }
// ↑ Outlook uses Word — flexbox, gap, box-shadow, border-radius all break
console.log(report.compatibility.scores["gmail-web"]);
// { score: 75, errors: 0, warnings: 5, info: 0 }
console.log(report.spam.score); // 100 (clean)
console.log(report.accessibility.score); // 88
console.log(report.size.clipped); // false (under Gmail's 102KB limit)Score too low? Fix it automatically:
import { generateAiFix, AI_FIX_SYSTEM_PROMPT } from "@emailens/engine";
const { code } = await generateAiFix({
originalHtml: html,
warnings: report.compatibility.warnings,
scores: report.compatibility.scores,
scope: "outlook-windows",
format: "jsx",
provider: async (prompt) => {
// Any LLM — Claude, GPT, etc.
const msg = await anthropic.messages.create({
model: "claude-sonnet-4-6",
max_tokens: 8192,
system: AI_FIX_SYSTEM_PROMPT,
messages: [{ role: "user", content: prompt }],
});
return msg.content[0].type === "text" ? msg.content[0].text : "";
},
});
// code → JSX with <Table> layout, VML roundrects, inline fallbacks10 analysis engines, one auditEmail() call.
- CSS compatibility — 255 CSS and HTML features tested across 21 email clients, with fix snippets and AI-powered auto-fix
- Content overflow — fixed widths wider than the email frame and unbreakable strings that force horizontal scrolling
- Visual bugs — gradients/background images with no color fallback (invisible content in Outlook) and fonts with no web-safe fallback, each with a concrete fix
- Spam scoring — 45+ signals modeled after SpamAssassin, CAN-SPAM, and GDPR
- Accessibility — WCAG contrast ratios, alt text, semantic structure, heading hierarchy
- Link validation — broken hrefs, insecure HTTP,
javascript:protocols, deceptive URLs - Image analysis — missing dimensions, oversized data URIs, tracking pixels, WebP/SVG format
- Inbox preview — subject/preheader truncation per client, Gmail clipping detection
- Domain authentication — SPF, DKIM, DMARC, MX, and BIMI DNS record validation
- Template variables — unresolved merge tags across 6 template systems (Handlebars, ERB, Mailchimp, etc.)
Every finding can carry its source position — line, column and offset, plus every place it occurs — so a result can be pointed at, annotated on a pull request, or fixed in place:
const report = auditEmail(html, { positions: true });
const w = report.compatibility.warnings[0];
`${file}:${w.loc.line}:${w.loc.column}`; // emails/welcome.html:42:8
w.locs.length; // every element it breaks onnpm install @emailens/engineThree entry points:
| Import | Description |
|---|---|
@emailens/engine |
Core analysis — CSS, spam, a11y, links, images, inbox preview, size, templates, AI fix |
@emailens/engine/compile |
JSX / MJML / Maizzle → HTML compilers |
@emailens/engine/server |
Node-only: DNS deliverability checks, SpamAssassin integration |
- Offline-first — runs entirely locally, no network calls required (except DNS deliverability checks)
- Unified audit — one function call returns CSS compatibility, spam, accessibility, links, images, inbox preview, size, and template checks
- Framework-aware — fix snippets tailored to React Email (JSX), MJML, and Maizzle
- AI-ready — structural issues get LLM-powered auto-fix with any provider (Claude, GPT, etc.)
- Programmable — TypeScript API, not a GUI — integrate into CI, editors, or build pipelines
| @emailens/engine | Litmus | Email on Acid | caniemail.com | |
|---|---|---|---|---|
| Local/offline | Yes | No | No | Data only |
| Programmatic API | Yes | Limited | No | No |
| CSS + Spam + A11y | Yes | Separate tools | Separate tools | CSS only |
| AI auto-fix | Yes | No | No | No |
| Open source | MIT | No | No | Yes (data) |
@emailens/engine sits in the QA / lint / scoring slot — it analyzes finished HTML. It's complementary to (not a replacement for) composition and inlining libraries.
| @emailens/engine | juice | email-comb | mjml | maizzle | |
|---|---|---|---|---|---|
| Purpose | QA / lint / score | CSS inliner | Unused CSS pruner | MJML → HTML | Tailwind → HTML |
| Per-client compatibility scoring | Yes | No | No | No | No |
| Spam / a11y / link / image analysis | Yes | No | No | No | No |
| AI-powered fix generation | Yes | No | No | No | No |
| Compose emails | Reads only | Reads only | Reads only | Yes | Yes |
| CSS inlining | No (pair with juice) | Yes | No | Yes (built-in) | Yes (built-in) |
A typical pipeline: write in mjml or maizzle → inline with juice → audit with @emailens/engine → ship.
| Client | ID | Category | Engine | Dark Mode |
|---|---|---|---|---|
| Gmail | gmail-web |
Webmail | Gmail Web | Yes |
| Gmail Android | gmail-android |
Mobile | Gmail Mobile | Yes |
| Gmail iOS | gmail-ios |
Mobile | Gmail Mobile | Yes |
| Outlook 365 | outlook-web |
Webmail | Outlook Web | Yes |
| Outlook (New) | outlook-windows |
Desktop | Outlook Web | Yes |
| Outlook Classic | outlook-windows-legacy |
Desktop | Microsoft Word | Yes |
| Outlook iOS | outlook-ios |
Mobile | Outlook Mobile | Yes |
| Outlook Android | outlook-android |
Mobile | Outlook Mobile | Yes |
| Outlook for Mac | outlook-macos |
Desktop | WebKit | Yes |
| Apple Mail | apple-mail-macos |
Desktop | WebKit | Yes |
| Apple Mail iOS | apple-mail-ios |
Mobile | WebKit | Yes |
| Yahoo Mail | yahoo-mail |
Webmail | Yahoo | Yes |
| Yahoo Mail Android | yahoo-mail-android |
Mobile | Yahoo | Yes |
| Yahoo Mail iOS | yahoo-mail-ios |
Mobile | Yahoo | Yes |
| Samsung Mail | samsung-mail |
Mobile | Samsung | Yes |
| Thunderbird | thunderbird |
Desktop | Gecko | No |
| HEY Mail | hey-mail |
Webmail | WebKit | Yes |
| Proton Mail | protonmail |
Webmail | Proton | Yes |
| AOL Mail | aol |
Webmail | AOL | Yes |
| Fastmail | fastmail |
Webmail | Fastmail | Yes |
| Superhuman | superhuman |
Desktop | Blink | Yes |
Full API reference: docs/API.md
Covers:
auditEmailandcreateSession— core analysis- Standalone analyzers (CSS, spam, links, accessibility, images, inbox preview, size, templates)
- DNS deliverability and SpamAssassin integration
- Client transforms and dark mode simulation
- Compile module (JSX, MJML, Maizzle)
- AI-powered fixes and token estimation
- Performance optimization guide
- Security considerations
- Full TypeScript type definitions
See ROADMAP.md for the full picture (shipped items + items under consideration with rationale).
Shipped: automated caniemail.com data sync · GitHub Actions integration via @emailens/cli and the Marketplace Action · AI-powered fix generation · compile module for JSX/MJML/Maizzle.
Considering: Outlook VML auto-generation · plugin system for custom analyzers · MJML/Maizzle source-level linting · ESLint plugin · spam corpus tuning · dark-mode accuracy tests.
Concrete bugs go in Issues. Open-ended ideas live in the roadmap.
Contributions are welcome! See CONTRIBUTING.md for architecture overview, setup instructions, and PR guidelines.
bun install && bun test # 719 testsOptional real-render validation — renders engine output in a real browser engine (free, no Litmus/Email on Acid needed). Off by default; needs a browser-capable machine:
bunx playwright install chromium
bun run test:renderCSS support data is auto-synced from caniemail.com. Other data (dark mode behavior, display limits, Superhuman overrides) is manually curated and tracked with verification dates.
bun run sync:caniemail # Refresh CSS support matrix from caniemail.com
bun run check:freshness # Flag stale data sources (exits 1 if any overdue)See CONTRIBUTING.md for full details on data sources and verification procedures.
MIT — Copyright 2025 Emailens
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