Definition
An email rendering engine is the underlying software component that interprets HTML and CSS code and displays the visual email layout to the recipient. Unlike web browsers, which use a small number of rendering engines (Blink, WebKit, Gecko), email clients use a diverse set of engines with vastly different levels of HTML and CSS support. The four primary rendering engines in email are WebKit (used by Apple Mail, iOS Mail, Samsung Mail, and Thunderbird), Blink (used by Gmail web and the new Outlook web), Microsoft Word's HTML engine (used by Outlook desktop 2007–2021), and Chromium (used by the new Outlook for Windows and Mac).
WebKit-based email clients offer the best CSS support, including CSS3 features such as animations, transforms, flexbox, and grid to varying degrees. Apple Mail on iOS and macOS supports responsive design through media queries and modern CSS layout techniques. Blink-based clients, including Gmail web and the newer Outlook web interfaces, also support modern CSS but with some notable gaps: Gmail strips <style> tags if CSS is not inlined and strips <head> sections entirely under certain conditions. The Word HTML engine used by Outlook desktop is the most limited, supporting only a subset of CSS 1.0 and 2.0 properties with table-based layout being the most reliable approach.
The diversity of rendering engines means that an email designed and tested in one engine may appear completely broken in another. Margin and padding behave differently, background images have inconsistent support, and font rendering varies significantly. Cross-engine testing is not optional; every email campaign should be tested in at least WebKit (Apple Mail), Blink (Gmail web), and Word HTML (Outlook desktop) before deployment. Tools such as Litmus and Email on Acid provide screenshots across all major engine combinations.
Best Practices
Test every email campaign in at least three rendering engine families: WebKit (Apple Mail), Blink (Gmail), and Word HTML (Outlook desktop). These three cover 95% of the email client landscape.
Code to the lowest common denominator engine (Outlook Word HTML) for core layout, then layer progressive enhancement for WebKit and Blink clients.
Use conditional MSO comments to provide Outlook-specific code paths. <!--[if mso]> and <!--[endif]--> target the Word engine without affecting other clients.
Avoid CSS properties that are unsupported in Outlook Word HTML: float (use table align instead), margin on block elements, background-position, border-radius (use VML for rounded corners), and display values beyond block, inline, none, and table.
Take advantage of WebKit-specific features in Apple Mail when your audience includes a high percentage of Apple users. Media queries, responsive breakpoints, and modern CSS selectors work reliably in WebKit.
Monitor rendering engine trends as clients migrate. New Outlook moving to Chromium and Gmail gradually improving CSS support means the rendering landscape is evolving.
Frequently Asked Questions
Gmail web uses Blink, the same engine as the Chrome browser. Gmail mobile apps use platform-native WebView components, which are WebKit-based on iOS and Blink-based on Android. The Inbox by Gmail and other Gmail variants also use Blink.
Microsoft designed Outlook 2007–2021 to use Word's rendering engine rather than Internet Explorer's (or Edge's) for security and consistency with Microsoft Word document processing. This decision dramatically limited CSS support and created the need for table-based email layouts. The new Outlook uses Chromium.
No. The new Outlook for Windows and Mac is built on Chromium and uses the Blink engine, which provides modern CSS support comparable to Gmail web. This is a significant improvement but also means that code targeting the old Word engine may not behave identically in the new Outlook.
Use Microsoft Office conditional statements: `` closes it. You can place Outlook-specific HTML, VML, or CSS inside these comments. This technique is essential for providing fallback code for the limited Word engine.
Use an email testing platform such as Litmus or Email on Acid that provides real screenshots across 70+ email client and engine combinations. These services render your email in actual client environments including Outlook Word, Gmail Blink, Apple Mail WebKit, and others, showing exactly how each engine displays your message.