Outlook email rendering can be inconsistent and unreliable due to several factors:
1. Different Rendering Engines: Outlook uses different rendering engines depending on the version. Most notably:
– Outlook 2007-2016 (Windows) uses the Microsoft Word rendering engine, which is designed for document formatting, not web standards like HTML and CSS. This engine is notoriously limited in its support for modern web design techniques.
– Outlook for Mac, Outlook 365 (web version), and the mobile app use different engines like Webkit, which support HTML and CSS much better. As a result, the same email may look different across these versions.
2. Limited CSS Support: Outlook’s rendering engine doesn’t fully support modern CSS. For example:
– Complex layouts like flexbox or grid are not supported.
– Issues with background images, padding, and margin.
– Limited support for web fonts—Outlook typically falls back to system fonts.
3. Table-Based Layouts: Many email clients, including Outlook, handle email layouts built with HTML tables more reliably than those built with divs or modern CSS. However, Outlook’s inconsistent handling of tables can still cause issues, especially with nested tables.
4. Varying DPI Settings: On high-DPI displays (such as those used in modern Windows devices), Outlook sometimes scales emails in unexpected ways, leading to rendering issues like blurry images or improper text scaling.
5. Conditional Comments: Outlook often requires the use of conditional comments (specific instructions for rendering only in Outlook) to handle its quirks. This increases development time and complexity.
6. Unsupported Interactive Elements: Interactive elements like forms, animations (via CSS or JavaScript), and embedded media may not work properly, leading to broken functionality.
To design emails that render well in Outlook, developers often use older techniques (like nested tables) and test extensively across different devices using our email preview testing.