Disable CSS pre-processing

What’s CSS pre-processing?

By default, before emails leave Customer.io, we pre-process their CSS styles with Premailer to improve rendering. Before sending, Premailer:

  • Converts CSS styles to inline attributes
  • Converts relative paths to absolute paths
  • Checks CSS properties against email client capabilities

You can find more details on Premailer’s website, as well as on the Github repository.

Why disable it?

If you’re using a custom email template which already has been pre-processed, running it through Premailer may cause some layout bugs. This most frequently happens with responsive email templates on mobile devices.

Disabling CSS pre-processing means that your CSS will stay exactly as specified in your template, avoiding these problems.

How to

First, you’ll need to switch to the code editor in your email composer:


Then, you’ll see the option to disable CSS pre-processing:


Once that box is checked, your email will be sent as created in the code editor — no CSS pre-processing!

Important Notes

Because this is an advanced feature, disabling CSS pre-processing is only available in the code editor within the composer. Once you exit the code viewer and enter the Rich Text editor, pre-processing will be enabled again:


If you have any questions about this, please let us know!

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