A phishing campaign using Office 365 voicemail lures users into visiting landing pages designed to steal their personal information or infect their computers with malware. The phishing emails delivered by the operators behind this series of attacks use the old trick of reversing some of the text elements in the source code and rendering forward within the email displayed to the target. This time it involves using Cascading Style Sheets (CSS) to hide the text from the Secure Email Gateway (SEG) while preserving the normal appearance to the human recipient.
Source: https://www.bleepingcomputer.com/news/security/office-365-phishing-uses-css-tricks-to-bypass-email-gateways/