Outlook sometimes skews HTML emails because all versions since Outlook 2007 use Microsoft Word to render HTML/CSS. Because of this, an email display may look slightly off in newer versions of Outlook from what you initially design and preview in Klaviyo.
Support for HTML Emails in Outlook 2007/2010/2013:
- No support for background images in divs and table cells
- No support for CSS float or position
- No support for text shadow
- Poor support for padding and margin
- Poor support for CSS width and height
- Problems with nested elements background colors
Optimizing an email specifically for recent versions of Outlook can take a lot of time and extensive testing, as there are no established guidelines. Doing this also requires making significant changes to how images are positioned, and this can skew how emails render across other email clients and devices.
This article from Litmus provides more detail on Rendering Differences in Microsoft Outlook Clients.