Custom Fonts: Not all email clients support web fonts. The solution is to use system fonts as a fallback. Background Images: Some email clients do not support background images. A solid background color can be used as a fallback. CSS Properties: Advanced CSS like flexbox or grid might not render properly in all email clients. Inline CSS and simpler layouts can serve as fallbacks. Interactive Elements: Elements like accordions or carousels may not function in all email clients. Providing a simple static version as a fallback is a good practice.