email-html-mjml — Responsive Email Developer

Generate valid, cross-client MJML 5.x templates and compile them to production-ready HTML. The primary goal is compatibility: Outlook (2013–365), Gmail (web/app), Apple Mail, and major mobile clients. Every output must be compilable with --config.validationLevel=strict and survive Gmail's 102KB clip limit.

Workflow

1. Gather requirements — Infer email type, brand colors, and content from the user's message and conversation context. Ask only for what is genuinely missing and blocking progress (e.g., no colors provided and the layout has branded sections). Never front-load a questionnaire.
2. Plan layout — Decide and announce the structure before writing code (single-column, 2-col grid, hero + content, etc.)
3. Load component references — Read the relevant file(s) from the Component Index below before writing any MJML
4. Generate MJML — Write complete, valid MJML starting from <mjml> with a full <mj-head>
5. Compile — Follow compilation.md. Run npx mjml with --config.minify=true
6. Deliver both files — Always output .mjml source AND compiled .html

9 Engineering Rules

1. Structural Integrity — All visual content MUST be in <mj-column> inside <mj-section>. Sections cannot be nested.
2. Responsive Defaults — Assume 600px width. Use <mj-group> to prevent mobile stacking for side-by-side elements (social bars, logo rows). Use <mj-section gutter="..."> for equal spacing between columns instead of padding tricks.
3. Outlook Compatibility — Use <mj-font> for web fonts (prevents Times New Roman fallback). Always provide a fallback stack (Arial, sans-serif). For <mj-section> background images, always set both background-size and a fallback background-color.
4. Gmail Optimization — Use inline="inline" on <mj-style> for custom CSS. Prefer component attributes (color, font-size) over CSS classes for critical styles.
5. Dark Mode — Include dark mode support when explicitly requested or when the email has a light background that would cause harsh forced-inversion. See the Dark Mode Pattern below.
6. Accessibility — Every <mj-image> MUST have alt. Always set <mj-title> (populates aria-label). Maintain WCAG 2.1 AA 4.5:1 contrast. For heading roles, use mj-html-attributes — direct role/aria-level attributes on mj-text are illegal under strict validation (see Accessibility Checklist below).
7. Styling Efficiency — Use <mj-attributes> with <mj-all>, component defaults, and <mj-class> to eliminate repetitive inline styles.
8. Hero Sections — Use <mj-hero> for full-bleed hero banners; it falls back to a regular section in unsupported clients. Avoid <mj-accordion> and <mj-carousel> — client support is too poor to be useful.
9. Templating Support — Plain interpolations ({{firstName}}) need no wrapping — they pass through text, attributes, mj-title and mj-preview untouched. Only block tags containing < or > ({% if x < 5 %}, {{#if a<b}}) need <!-- htmlmin:ignore --> protection, or minify silently corrupts them. An <mj-raw> between content components inside a column emits its content between </tr> and <tr> — invalid markup; put mj-raw between sections, and keep templating tags out of <mj-style> (see compilation.md).

Critical Gotchas

MJML 5 mj-include is disabled by default:

• <mj-include> tags are silently ignored unless includes are explicitly enabled — a deliberate security change from MJML 4
• CLI: add --config.allowIncludes true. Node API: pass ignoreIncludes: false
• Scope allowed directories with --config.includePath (CLI) or includePath (Node) — only MJML, HTML, and CSS includes are supported
• A partial outside the template's own directory is denied unless its directory is allowlisted
• --config.validationLevel=strict does not catch this — a dropped include still exits 0 and still writes the .html. A clean compile is NOT proof the include landed; grep the output for known partial content before delivering
• If a compiled template is missing content you expected from an include, check for this first

Outlook:

• Background images: VML only generated for <mj-section> and <mj-hero> — nowhere else
• Background positioning: keyword values only (top, center, bottom) — pixel values ignored
• Always pair background-repeat="no-repeat" with explicit background-size
• Font fallback: <mj-font> hides @font-face from Outlook via MSO conditional comments

Gmail:

• Use component attributes for critical layout — CSS classes may be stripped
• 102KB clip: always compile with --config.minify=true

iOS / Android stacking:

• MJML 5's minify no longer strips whitespace between tags — htmlnano collapses runs to a single space where html-minifier removed them (verified: v4 emits <![endif]--><div class="mj-column-per-50", v5 emits <![endif]--> <div ...). The anti-stacking defence that still works is the font-size:0px MJML puts on the parent <td>, which is emitted in both versions
• To restore v4 whitespace stripping: --config.minifyOptions '{"collapseWhitespace":"all"}' (JSON string form required)
• Whitespace between tags causes stacking even inside <mj-group>

Vertical-align bug:

• If any column in a section sets vertical-align, ALL columns in that section must explicitly set it

JavaScript:

• JS is completely blocked in all email clients (Gmail, Outlook, Apple Mail, iOS Mail). No onclick, no clipboard API, no interactivity of any kind. Interactive-looking elements (copy buttons, toggles) are purely decorative.

Dark Mode Pattern

<mj-head>
  <mj-raw>
    <meta name="color-scheme" content="light dark">
    <meta name="supported-color-schemes" content="light dark">
  </mj-raw>

  <!-- Light logo visible by default; dark logo hidden -->
  <mj-style inline="inline">
    .dark-logo { display: none !important; }
  </mj-style>

  <!-- Dark mode overrides -->
  <mj-style>
    @media (prefers-color-scheme: dark) {
      .light-logo { display: none !important; }
      .dark-logo  { display: block !important; }
    }
  </mj-style>
</mj-head>

Safe neutrals: #121212 (not #000000) and #F1F1F1 (not #FFFFFF) — prevents jarring forced inversions.

Accessibility Checklist

• <mj-title> is set (screen reader email label + aria-label)
• lang attribute on root <mjml> tag
• alt on every <mj-image> and <mj-social-element>
• Heading role set via mj-html-attributes (NOT as a direct attribute on mj-text):

  <!-- In mj-head -->
  <mj-html-attributes>
    <mj-selector path=".email-heading div">
      <mj-html-attribute name="role">heading</mj-html-attribute>
      <mj-html-attribute name="aria-level">1</mj-html-attribute>
    </mj-selector>
  </mj-html-attributes>
  <!-- On the component -->
  <mj-text css-class="email-heading" ...>Heading text</mj-text>
  

  Copy
• 4.5:1 contrast ratio on all text/background pairs
• No text baked into images — always use live <mj-text> blocks

Component Index

Before writing any MJML, read the component file(s) for the components you'll use.

General reference (hierarchy, ending tags, validation, width math, Gmail clip): mjml-reference.md

Compilation

Read compilation.md for the full workflow. Key command:

npx mjml template.mjml -o dist/template.html --config.minify=true --config.validationLevel=strict

Hard rules:

• Never npm install -g mjml
• Always use npx or ./node_modules/.bin/mjml
• If mjml not in package.json, suggest npm install -D mjml

Examples

All four compile clean under --config.validationLevel=strict --config.minify=true on MJML 5.4.0.

All copy, image URLs, and brands in these files are placeholders — replace them; do not ship them as-is.

Output

Always deliver:

1. <name>.mjml — complete MJML source (editable, version-controllable)
2. <name>.html — compiled output (production-ready, send via ESP)

Name files after the email type: welcome.mjml, promo-sale.mjml, order-confirmation.mjml