Dev guide to perfect bulletproof background images in Outlook (The VML trick, simplified)

Summary: Outlook uses Microsoft Word’s rendering engine, which doesn’t support CSS background images. The traditional fix is using VML code. The modern solution is letting Beefree SDK handle rendering automatically through its /html endpoint.

I know the feeling: you’ve coded a beautiful email design with an eye-catching background image and it’s rendering perfectly in your test environment. You check everything from Apple Mail to Gmail and it not only looks good, it’s fully responsive on mobile devices. 

Unfortunately, when you open your test email in Outlook, your jaw drops. The background image is gone. Just… gone and replaced by some random fallback color. Your perfectly crafted image section is now a blank slate. What happened?

Fortunately, this probably isn’t a bug in your code, and you’re not the first developer to stare at the screen in disbelief. This post will show you exactly why this happens, then share with you the modern way to architect your workflow so you never have to worry about it again.

The “why”: Understanding why Outlook breaks your email background images

The first thing to really digest is the fact Outlook is not a web browser. When running on Windows, Outlook doesn’t use a modern engine like WebKit or Blink to render HTML. It uses the Microsoft Word rendering engine.

Think about that for a second. Your carefully-crafted HTML and CSS are being interpreted by a word processor. This is the root cause of countless email development frustrations. The Word engine was built to lay out printed documents, not responsive web content. As a result, its support for modern web standards like background-image CSS properties on block elements is notoriously poor.

So, how do we force a word processor to display a background image? We have to do a little digging, then use a language it understands.

The old way: a painful dive into VML

The secret handshake that email developers have been forced to use for years is called VML (Vector Markup Language). VML is an XML-based language from the Internet Explorer 5 era that Microsoft Word (and therefore Outlook) still understands and can render.

To get your background image to show up, you can’t just rely on your CSS. You have to embed a chunk of VML code inside conditional comments that only Outlook will read. It’s a solution that works, but honestly, it’s not for the faint of heart.

Here is the standard, full block of code required to make this work:

Let’s break down this monster:

• Conditional Comments: The `` tags ensure that only Microsoft Office (mso) Works 9 and newer will see and render the VML code inside. Every other email client will ignore it.
• <v:rect>: This is a VML element that creates a rectangular vector shape. We set its style to match the dimensions of our table cell.
• <v:fill>: This element fills the rectangle. We provide our image source and a fallback color.
• <v:textbox>: This is the container for our actual HTML content (text, buttons, etc.) that needs to sit on top of the background image.

The takeaway here is clear: this works, but it's verbose and rather messy to maintain. For every background image, you’d have to copy, paste, and carefully edit this block of legacy code.

A better way: abstracting complexity

At this point you realize you’d never want to write a line of VML again. Any modern developer workflow should allow you to focus on your design’s intent, not on writing boilerplate for outdated rendering engines.

The way forward is an architectural shift. But the shift isn't from hand-coded HTML to hand-coded JSON. The shift is from manual rendering to delegated rendering. Your job isn't to be an email-coding expert; it's to provide a great experience for your users and get a reliable asset at the end.

This is the exact problem the Beefree SDK workflow solves. Here’s how it works:

1. Your end-user builds their design in the no-code builder you've embedded in your application. They are visually adding background images, text, and buttons.
2. When they save, the builder provides a clean, predictable JSON object. This is the design's "source of truth."
3. As the developer, you don't have to parse, manipulate, or even understand that JSON. You simply take that object and send it to our Content Services API, specifically the /html endpoint.

That one API call is the entire solution.

Our specialized rendering engine receives that JSON blueprint. It understands the target is email. It then generates the complete, responsive, bulletproof HTML for you by building out the VML code for Outlook while also providing the standard CSS background-image property for every other client.

You don't write VML. You don't have to maintain complex templates. You let your users build visually, and you make one API call to get perfect, production-ready HTML that just works. In short, Beefree SDK gives you a simple, declarative workflow; we handle the complex, bulletproof output without any code wrangling on your end.

Conclusion: focus on what matters

We're the email rendering experts, but we want you to win at email as well! By embedding the best-in-class Beefree SDK, you can stop wrestling with email client bugs and legacy code. Instead, you can architect your content workflow around structured data like JSON and let specialized tools handle the complex rendering.

Basically, build features. Don’t play with tables <tr>No more</tr>.

Ready to stop writing VML for good? Explore how to manage row and content styling in the Beefree SDK Documentation.