MJML compilation is the process of converting MJML markup into email-ready HTML. During compilation, MJML transforms high-level layout tags into nested tables, inlines styles, and produces markup that is more likely to render consistently across email clients. Compilation is the bridge between developer-friendly authoring and real-world email constraints.
What Happens During Compilation
Compilation also affects how you store templates. Some teams store MJML source and compile on publish, while others store compiled HTML as the canonical artifact. Whichever approach you choose, keep version IDs so you can reproduce the exact output that was sent.
Compilation expands MJML components into the detailed HTML required for compatibility. It often generates tables, wraps elements for spacing, and applies inline styles to reduce reliance on external CSS. In many workflows, the compiled output becomes the artifact that gets stored and sent, even if the source remains MJML. That is why it is important to validate the output, not only the input.
Inlining and Output Quality
One of the most important compilation outcomes is inline CSS. Many clients handle inline styles more reliably than embedded stylesheets. However, inlining can increase output size, which can contribute to clipped email. This is a common tradeoff: more reliable styling versus larger HTML payloads. You can reduce bloat by trimming unused components and avoiding overly repetitive structures.
Validation and Debugging Strategy
When an email breaks, debug the compiled HTML. Use HTML validation to catch malformed output and structural issues. Keep a small set of known-good templates and compare diffs when upgrading MJML versions or changing custom components. This makes regressions easier to isolate and reduces the risk of shipping broken templates after a library upgrade.
MJML Compilation and Topol
Topol helps teams manage predictable email output and structured editing workflows, reducing the fragility that can come from large compiled templates and making validation easier to operationalize. Learn more at Topol or sign up at Topol signup.