АТ
Unfortunately Outlook supports something roughly equivalent to IE5 compatible HTML. It's really terrible. Here's a detailed MSDN article on the Word 2007 HTML and CSS Rendering Capabilities in Outlook 2007, which I don't believe changed much for Outlook 2010.
Honestly, the only way I've been able to get outlook HTML to look the way I want is to hand generate the HTML using roughly HTML2 standard tags and properties and not using CSS at all. Some CSS renders, but it's really hit or miss.
https://stackoverflow.com/questions/8310609/css-styling-wont-work-in-outlook-2010
https://docs.microsoft.com/en-us/previous-versions/office/developer/office-2007/aa338201(v=office.12)?redirectedfrom=MSDN