Creating HTML emails that are visually appealing, functional, and responsive is vital for effective email marketing in Adobe Marketo. Since email coding differs significantly from standard web design, special considerations are necessary to ensure compatibility across various email clients. Below are the best practices to follow when designing HTML emails for Marketo.

1. Understand Email Client Constraints

Use basic HTML (HTML 4.01 or XHTML 1.0 Transitional).
Avoid JavaScript or interactive elements, as most email clients do not support them.
Test your emails in multiple email clients to identify rendering issues early.

2. Use Table-Based Layouts

Email design still favors table-based layouts due to better support in email clients:

Use nested tables to create grid-based layouts.
Apply inline styles to table elements for consistent design across platforms.

<table width="600" cellpadding="0" cellspacing="0" border="0">
  <tr>
    <td align="center" style="font-family: Arial, sans-serif; font-size: 16px; color: #333333;">
      Welcome to our newsletter!
    </td>
  </tr>
</table>

3. Apply Inline CSS

Many email clients strip out <style> tags, so inline CSS is essential:

Avoid external stylesheets and use inline styling instead.
Stick to basic CSS properties supported by most email clients.

<td style="background-color: #f4f4f4; padding: 20px; font-size: 14px; line-height: 1.5;">
  Thank you for subscribing!
</td>

4. Keep the Code Clean and Simple

Avoid shorthand CSS (e.g., margin: 10px 20px;).
Use absolute paths for images and links.
Minimize complex code to reduce rendering issues.

5. Design for Responsiveness

A majority of users check emails on mobile devices, so responsive design is critical:

Use media queries to adapt styles for smaller screens.
Set a maximum width (e.g., 600px) for your email layout and ensure it scales down gracefully.

<style>
  @media only screen and (max-width: 600px) {
    .responsive {
      width: 100% !important;
      padding: 10px !important;
    }
  }
</style>
<table class="responsive" width="600" cellpadding="0" cellspacing="0">
  <tr>
    <td style="padding: 20px;">Responsive content here</td>
  </tr>
</table>

6. Incorporate Marketo Tokens

Adobe Marketo allows dynamic personalization through tokens:

Use tokens like {{lead.FirstName}} to personalize content.
Incorporate branding tokens for consistent logos or company names.

<td style="font-size: 16px;">
  Hi {{lead.FirstName}}, welcome to our community!
</td>

7. Add ALT Text to Images

Ensure accessibility by providing descriptive ALT text for all images:

<img src="https://example.com/logo.png" alt="Company Logo" width="100" style="display: block;">

8. Test Across Email Clients

Testing is crucial to identify potential rendering issues:

Use tools like Litmus or Email on Acid to preview your email design across devices and email clients.
Verify functionality in both desktop and mobile versions of popular email platforms.

9. Provide Fallbacks for Unsupported Features

Some email clients, such as older versions of Outlook, have limited CSS support. Use conditional comments to provide fallback styles:

<!--[if mso]>
  <style>
    .button {
      background-color: #007BFF !important;
    }
  </style>
<![endif]-->

10. Ensure Compliance

Your emails must comply with regulations such as GDPR and CAN-SPAM:

Include a visible unsubscribe link.
Provide a physical mailing address in the footer.
Avoid misleading subject lines or content.

<footer style="font-size: 12px; color: #666;">
  You are receiving this email because you subscribed to our newsletter.<br>
  <a href="{{system.unsubscribeLink}}" style="color: #007BFF;">Unsubscribe</a>
</footer>

11. Follow Accessibility Standards

Use semantic HTML tags (e.g., <h1> for headings).
Add ARIA labels for interactive elements.
Ensure sufficient color contrast for text and backgrounds.

12. Optimize for Speed

Compress images without sacrificing quality.
Avoid embedding large videos or attachments.
Minify your HTML and CSS code for faster loading times.

Best Practices for HTML Email Coding in Marketo