For AI agents: llms.txt is a short guide that sends you to the rules your problem needs, and rules.json holds every rule of this playbook as JSON.

Applies when

The project it sends email.

Cost

Open source. MIT licence, free to use in any project.

How to do it

MJML 1.0.0 shipped on 29 January 2016, and 5.4.1 on 10 September 2026. npm counted about 1.1 million downloads in the week to 18 October 2026. Its engine turns short tags into the responsive HTML that email clients need, so nobody writes layout tables by hand. Compile it with Node.js or its command line, then render the output with any template engine. The output still needs what Build layout that Outlook can show asks, so test it in the clients. Choose something else when your email templates are React components. React Email's components package reached 1.0.0 in November 2025, so record the exception.

Examples

Using these examples: they show one way to follow this rule. Keep the behaviour and the check, not the syntax. Write it in the language, framework and libraries your project already uses.

JavaScript example

import { readFile, writeFile } from 'node:fs/promises';
import mjml2html from 'mjml';

export async function compileTemplate(name) {
  const source = await readFile(`emails/${name}.mjml`, 'utf8');
  const { html } = await mjml2html(source, { validationLevel: 'strict' });
  await writeFile(`build/emails/${name}.html`, html);
}

Python example

from jinja2 import Environment, FileSystemLoader, select_autoescape

emails = Environment(loader=FileSystemLoader("build/emails"), autoescape=select_autoescape(["html"]))

def render_receipt(order) -> str:
    return emails.get_template("receipt.html").render(order=order)

How to check

The build compiles every .mjml file with validationLevel: 'strict' and fails on any error. No sent template is hand-written HTML.

Sources

• MJML documentation

Related rules

• Build layout that Outlook can show in Email