diff --git a/lib/Epub/Epub/blocks/BlockStyle.h b/lib/Epub/Epub/blocks/BlockStyle.h
index a5a616bf9..4166fceb6 100644
--- a/lib/Epub/Epub/blocks/BlockStyle.h
+++ b/lib/Epub/Epub/blocks/BlockStyle.h
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <algorithm>
 #include <cstdint>
 
 #include "Epub/css/CssStyle.h"
@@ -8,6 +9,12 @@
  * BlockStyle - Block-level styling properties
  */
 struct BlockStyle {
+  // Upper bound (in em) for any single side's horizontal margin or padding.
+  // Some EPUBs apply huge em-based insets to chapter-opener classes; without a
+  // cap, effectiveWidth collapses to 1-2 words per line and justification dumps
+  // the remaining space into a single gap.
+  static constexpr float MAX_HORIZONTAL_INSET_EM = 2.0f;
+
   CssTextAlign alignment = CssTextAlign::Justify;
 
   // Spacing (in pixels)
@@ -68,16 +75,17 @@ struct BlockStyle {
                                  const uint16_t viewportWidth = 0) {
     BlockStyle blockStyle;
     const float vw = viewportWidth;
+    const auto maxHorizontalInsetPx = static_cast<int16_t>(emSize * MAX_HORIZONTAL_INSET_EM);
     // Resolve all CssLength values to pixels using the current font's em size and viewport width
     blockStyle.marginTop = cssStyle.marginTop.toPixelsInt16(emSize, vw);
     blockStyle.marginBottom = cssStyle.marginBottom.toPixelsInt16(emSize, vw);
-    blockStyle.marginLeft = cssStyle.marginLeft.toPixelsInt16(emSize, vw);
-    blockStyle.marginRight = cssStyle.marginRight.toPixelsInt16(emSize, vw);
+    blockStyle.marginLeft = std::min(cssStyle.marginLeft.toPixelsInt16(emSize, vw), maxHorizontalInsetPx);
+    blockStyle.marginRight = std::min(cssStyle.marginRight.toPixelsInt16(emSize, vw), maxHorizontalInsetPx);
 
     blockStyle.paddingTop = cssStyle.paddingTop.toPixelsInt16(emSize, vw);
     blockStyle.paddingBottom = cssStyle.paddingBottom.toPixelsInt16(emSize, vw);
-    blockStyle.paddingLeft = cssStyle.paddingLeft.toPixelsInt16(emSize, vw);
-    blockStyle.paddingRight = cssStyle.paddingRight.toPixelsInt16(emSize, vw);
+    blockStyle.paddingLeft = std::min(cssStyle.paddingLeft.toPixelsInt16(emSize, vw), maxHorizontalInsetPx);
+    blockStyle.paddingRight = std::min(cssStyle.paddingRight.toPixelsInt16(emSize, vw), maxHorizontalInsetPx);
 
     // For textIndent: if it's a percentage we can't resolve (no viewport width),
     // leave textIndentDefined=false so the EmSpace fallback in applyParagraphIndent() is used