The Microsoft Outlook App for desktop at times skews HTML emails because all versions since Outlook 2007 use Microsoft Word to render HTML/CSS. Due to this, an email display may look a little distorted in newer versions of Outlook from what you originally designed and previewed in any of your Email Service Providers.
In general, you may encounter issues with the following 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
As there are no established guidelines, optimizing and perfecting an email specifically for recent versions of Outlook requires a lot of time and extensive testing. It also requires making significant changes to how the images are positioned, and this can result in skewing how emails render across other email clients and devices.
For more information regarding this topic, read Litmus' blog article on Rendering Differences in Microsoft Outlook Clients.