Understanding CSS Optimization

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Ensuring your emails hit the inbox matters immensely for the success of your email marketing. Likewise, ensuring that your emails aren’t clipped is crucial for customers to get all the information in your messages. CSS optimization is one of the many ways that you can make sure your emails are not getting clipped in the inbox.

CSS Optimization

When you build emails in your account, Klaviyo uses something called embedded styles in order to add in the branding that you select in the Styles, Block Styles, and Block tabs. By using these to customize your emails, your branded emails have the smallest file size possible to keep them from getting clipped. The largest inbox providers — Gmail, Yahoo, Apple, Outlook, AOL — all accept and render embedded styles.

Smaller inbox providers do not render embedded styles. If you find a large number of your customers are using the inbox providers below or another provider that does not render embedded styles, consider unchecking the CSS Optimization box so your emails look as you designed them.


Know that, if you uncheck this box, you run the risk of your emails being clipped in other inbox providers, so test your emails before you send them.


Common Inbox Providers that Do Not Support Embedded Styles

 Australia telstra.com.au, telstra.com,  bigpond.net.au, bigpond.com
 Russia mail.ru
 Germany/EU gmx.com, t-online.de
Brazil  bol.com

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