Fix Default Image Blocking Layouts in Mozilla Thunderbird — Klaviyo Guide for Client Rendering Engine Anomalies
Email client rendering variations are caused by differences in parser engines. Legacy Outlook engines ignore modern block models, forcing layouts to stack vertically. Below is the targeted integration guide to fix Default Image Blocking Layouts inside Mozilla Thunderbird templates when using Klaviyo.
Technical Breakdown
Mozilla Thunderbird uses the Gecko which lacks support for standard rendering rules. This causes Default Image Blocking Layouts elements to break or collapse when rendering templates.
Klaviyo Behaviors
Klaviyo's drag-and-drop template parser automatically wraps all user-defined block sections in proprietary 'kl-part' class selectors. When rendering templates inside Microsoft Word engines (Outlook 2007-2021), Klaviyo's nested flex fallback blocks can cause structural tables to collapse or shift columns. You must use explicit table widths to override Klaviyo's styling wraps. Test layout code regressions side-by-side using the Campaign Diff Comparator before pushing visual style changes to your production templates.
Remediation Code Block
To resolve this, always define explicit width and height attributes on your images to preserve their layout boxes when blocked. Add robust alternative (ALT) text styled with CSS font-sizes and colors, and encode vital brand logos directly as Base64 data URIs. In your Klaviyo campaign manager, make sure you configure your links to run via {% unsubscribe %}.
<img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAADIA..." width="150" height="40" alt="Company Logo" style="display: block; font-family: sans-serif; font-size: 14px; color: #111111; font-weight: bold; border: 0;">Live Code Sandbox Test
How to Deploy inside Klaviyo
Copy the fixed VML or table wrapper code block from the remediation box above.
Open your Klaviyo template editor. Drag an 'HTML' block directly into your layout timeline (do not use a standard text box). Clear the default skeleton content inside the Klaviyo markup editor and paste the RenderOps corrected inline code.