feat: Add CSS parsing and CSS support in EPUBs (#411)

## Summary * **What is the goal of this PR?** - Adds basic CSS parsing to EPUBs and determine the CSS rules when rendering to the screen so that text is styled correctly. Currently supports bold, underline, italics, margin, padding, and text alignment ## Additional Context - My main reason for wanting this is that the book I'm currently reading, Carl's Doomsday Scenario (2nd in the Dungeon Crawler Carl series), relies _a lot_ on styled text for telling parts of the story. When text is bolded, it's supposed to be a message that's rendered "on-screen" in the story. When characters are "chatting" with each other, the text is bolded and their names are underlined. Plus, normal emphasis is provided with italicizing words here and there. So, this greatly improves my experience reading this book on the Xteink, and I figured it was useful enough for others too. - For transparency: I'm a software engineer, but I'm mostly frontend and TypeScript/JavaScript. It's been _years_ since I did any C/C++, so I would not be surprised if I'm doing something dumb along the way in this code. Please don't hesitate to ask for changes if something looks off. I heavily relied on Claude Code for help, and I had a lot of inspiration from how [microreader](https://github.com/CidVonHighwind/microreader) achieves their CSS parsing and styling. I did give this as good of a code review as I could and went through everything, and _it works on my machine_ 😄 ### Before ![IMG_6271](https://github.com/user-attachments/assets/dba7554d-efb6-4d13-88bc-8b83cd1fc615) ![IMG_6272](https://github.com/user-attachments/assets/61ba2de0-87c9-4f39-956f-013da4fe20a4) ### After ![IMG_6268](https://github.com/user-attachments/assets/ebe11796-cca9-4a46-b9c7-0709c7932818) ![IMG_6269](https://github.com/user-attachments/assets/e89c33dc-ff47-4bb7-855e-863fe44b3202) --- ### AI Usage Did you use AI tools to help write this code? **YES**, Claude Code
2026-02-05 21:28:10 +11:00
parent db659f3ea2
commit 2cf799f45b
21 changed files with 1623 additions and 126 deletions
@@ -22,6 +22,9 @@ constexpr int NUM_BOLD_TAGS = sizeof(BOLD_TAGS) / sizeof(BOLD_TAGS[0]);
 const char* ITALIC_TAGS[] = {"i", "em"};
 constexpr int NUM_ITALIC_TAGS = sizeof(ITALIC_TAGS) / sizeof(ITALIC_TAGS[0]);

+const char* UNDERLINE_TAGS[] = {"u", "ins"};
+constexpr int NUM_UNDERLINE_TAGS = sizeof(UNDERLINE_TAGS) / sizeof(UNDERLINE_TAGS[0]);
+
 const char* IMAGE_TAGS[] = {"img"};
 constexpr int NUM_IMAGE_TAGS = sizeof(IMAGE_TAGS) / sizeof(IMAGE_TAGS[0]);

@@ -40,17 +43,51 @@ bool matches(const char* tag_name, const char* possible_tags[], const int possib
  return false;
 }

+bool isHeaderOrBlock(const char* name) {
+  return matches(name, HEADER_TAGS, NUM_HEADER_TAGS) || matches(name, BLOCK_TAGS, NUM_BLOCK_TAGS);
+}
+
+// Update effective bold/italic/underline based on block style and inline style stack
+void ChapterHtmlSlimParser::updateEffectiveInlineStyle() {
+  // Start with block-level styles
+  effectiveBold = currentCssStyle.hasFontWeight() && currentCssStyle.fontWeight == CssFontWeight::Bold;
+  effectiveItalic = currentCssStyle.hasFontStyle() && currentCssStyle.fontStyle == CssFontStyle::Italic;
+  effectiveUnderline =
+      currentCssStyle.hasTextDecoration() && currentCssStyle.textDecoration == CssTextDecoration::Underline;
+
+  // Apply inline style stack in order
+  for (const auto& entry : inlineStyleStack) {
+    if (entry.hasBold) {
+      effectiveBold = entry.bold;
+    }
+    if (entry.hasItalic) {
+      effectiveItalic = entry.italic;
+    }
+    if (entry.hasUnderline) {
+      effectiveUnderline = entry.underline;
+    }
+  }
+}
+
 // flush the contents of partWordBuffer to currentTextBlock
 void ChapterHtmlSlimParser::flushPartWordBuffer() {
-  // determine font style
+  // Determine font style from depth-based tracking and CSS effective style
+  const bool isBold = boldUntilDepth < depth || effectiveBold;
+  const bool isItalic = italicUntilDepth < depth || effectiveItalic;
+  const bool isUnderline = underlineUntilDepth < depth || effectiveUnderline;
+
+  // Combine style flags using bitwise OR
  EpdFontFamily::Style fontStyle = EpdFontFamily::REGULAR;
-  if (boldUntilDepth < depth && italicUntilDepth < depth) {
-    fontStyle = EpdFontFamily::BOLD_ITALIC;
-  } else if (boldUntilDepth < depth) {
-    fontStyle = EpdFontFamily::BOLD;
-  } else if (italicUntilDepth < depth) {
-    fontStyle = EpdFontFamily::ITALIC;
+  if (isBold) {
+    fontStyle = static_cast<EpdFontFamily::Style>(fontStyle | EpdFontFamily::BOLD);
  }
+  if (isItalic) {
+    fontStyle = static_cast<EpdFontFamily::Style>(fontStyle | EpdFontFamily::ITALIC);
+  }
+  if (isUnderline) {
+    fontStyle = static_cast<EpdFontFamily::Style>(fontStyle | EpdFontFamily::UNDERLINE);
+  }
+
  // flush the buffer
  partWordBuffer[partWordBufferIndex] = '\0';
  currentTextBlock->addWord(partWordBuffer, fontStyle);
@@ -58,17 +95,20 @@ void ChapterHtmlSlimParser::flushPartWordBuffer() {
 }

 // start a new text block if needed
-void ChapterHtmlSlimParser::startNewTextBlock(const TextBlock::Style style) {
+void ChapterHtmlSlimParser::startNewTextBlock(const BlockStyle& blockStyle) {
  if (currentTextBlock) {
    // already have a text block running and it is empty - just reuse it
    if (currentTextBlock->isEmpty()) {
-      currentTextBlock->setStyle(style);
+      // Merge with existing block style to accumulate CSS styling from parent block elements.
+      // This handles cases like <div style="margin-bottom:2em"><h1>text</h1></div> where the
+      // div's margin should be preserved, even though it has no direct text content.
+      currentTextBlock->setBlockStyle(currentTextBlock->getBlockStyle().getCombinedBlockStyle(blockStyle));
      return;
    }

    makePages();
  }
-  currentTextBlock.reset(new ParsedText(style, extraParagraphSpacing, hyphenationEnabled));
+  currentTextBlock.reset(new ParsedText(extraParagraphSpacing, hyphenationEnabled, blockStyle));
 }

 void XMLCALL ChapterHtmlSlimParser::startElement(void* userData, const XML_Char* name, const XML_Char** atts) {
@@ -80,13 +120,30 @@ void XMLCALL ChapterHtmlSlimParser::startElement(void* userData, const XML_Char*
    return;
  }

+  // Extract class and style attributes for CSS processing
+  std::string classAttr;
+  std::string styleAttr;
+  if (atts != nullptr) {
+    for (int i = 0; atts[i]; i += 2) {
+      if (strcmp(atts[i], "class") == 0) {
+        classAttr = atts[i + 1];
+      } else if (strcmp(atts[i], "style") == 0) {
+        styleAttr = atts[i + 1];
+      }
+    }
+  }
+
+  auto centeredBlockStyle = BlockStyle();
+  centeredBlockStyle.textAlignDefined = true;
+  centeredBlockStyle.alignment = CssTextAlign::Center;
+
  // Special handling for tables - show placeholder text instead of dropping silently
  if (strcmp(name, "table") == 0) {
    // Add placeholder text
-    self->startNewTextBlock(TextBlock::CENTER_ALIGN);
+    self->startNewTextBlock(centeredBlockStyle);

    self->italicUntilDepth = min(self->italicUntilDepth, self->depth);
-    // Advance depth before processing character data (like you would for a element with text)
+    // Advance depth before processing character data (like you would for an element with text)
    self->depth += 1;
    self->characterData(userData, "[Table omitted]", strlen("[Table omitted]"));

@@ -111,9 +168,9 @@ void XMLCALL ChapterHtmlSlimParser::startElement(void* userData, const XML_Char*

    Serial.printf("[%lu] [EHP] Image alt: %s\n", millis(), alt.c_str());

-    self->startNewTextBlock(TextBlock::CENTER_ALIGN);
+    self->startNewTextBlock(centeredBlockStyle);
    self->italicUntilDepth = min(self->italicUntilDepth, self->depth);
-    // Advance depth before processing character data (like you would for a element with text)
+    // Advance depth before processing character data (like you would for an element with text)
    self->depth += 1;
    self->characterData(userData, alt.c_str(), alt.length());

@@ -141,43 +198,113 @@ void XMLCALL ChapterHtmlSlimParser::startElement(void* userData, const XML_Char*
    }
  }

-  if (matches(name, HEADER_TAGS, NUM_HEADER_TAGS)) {
-    self->startNewTextBlock(TextBlock::CENTER_ALIGN);
-    self->boldUntilDepth = std::min(self->boldUntilDepth, self->depth);
-    self->depth += 1;
-    return;
+  // Compute CSS style for this element
+  CssStyle cssStyle;
+  if (self->cssParser) {
+    // Get combined tag + class styles
+    cssStyle = self->cssParser->resolveStyle(name, classAttr);
+    // Merge inline style (highest priority)
+    if (!styleAttr.empty()) {
+      CssStyle inlineStyle = CssParser::parseInlineStyle(styleAttr);
+      cssStyle.applyOver(inlineStyle);
+    }
  }

-  if (matches(name, BLOCK_TAGS, NUM_BLOCK_TAGS)) {
+  const float emSize = static_cast<float>(self->renderer.getLineHeight(self->fontId)) * self->lineCompression;
+  const auto userAlignment = static_cast<CssTextAlign>(self->paragraphAlignment);
+
+  if (matches(name, HEADER_TAGS, NUM_HEADER_TAGS)) {
+    self->currentCssStyle = cssStyle;
+    self->startNewTextBlock(BlockStyle::fromCssStyle(cssStyle, emSize, userAlignment));
+    self->boldUntilDepth = std::min(self->boldUntilDepth, self->depth);
+    self->updateEffectiveInlineStyle();
+  } else if (matches(name, BLOCK_TAGS, NUM_BLOCK_TAGS)) {
    if (strcmp(name, "br") == 0) {
      if (self->partWordBufferIndex > 0) {
        // flush word preceding <br/> to currentTextBlock before calling startNewTextBlock
        self->flushPartWordBuffer();
      }
-      self->startNewTextBlock(self->currentTextBlock->getStyle());
-      self->depth += 1;
-      return;
+      self->startNewTextBlock(self->currentTextBlock->getBlockStyle());
+    } else {
+      self->currentCssStyle = cssStyle;
+      self->startNewTextBlock(BlockStyle::fromCssStyle(cssStyle, emSize, userAlignment));
+      self->updateEffectiveInlineStyle();
+
+      if (strcmp(name, "li") == 0) {
+        self->currentTextBlock->addWord("\xe2\x80\xa2", EpdFontFamily::REGULAR);
+      }
    }
-
-    self->startNewTextBlock(static_cast<TextBlock::Style>(self->paragraphAlignment));
-    if (strcmp(name, "li") == 0) {
-      self->currentTextBlock->addWord("\xe2\x80\xa2", EpdFontFamily::REGULAR);
+  } else if (matches(name, UNDERLINE_TAGS, NUM_UNDERLINE_TAGS)) {
+    self->underlineUntilDepth = std::min(self->underlineUntilDepth, self->depth);
+    // Push inline style entry for underline tag
+    StyleStackEntry entry;
+    entry.depth = self->depth;  // Track depth for matching pop
+    entry.hasUnderline = true;
+    entry.underline = true;
+    if (cssStyle.hasFontWeight()) {
+      entry.hasBold = true;
+      entry.bold = cssStyle.fontWeight == CssFontWeight::Bold;
    }
-
-    self->depth += 1;
-    return;
-  }
-
-  if (matches(name, BOLD_TAGS, NUM_BOLD_TAGS)) {
+    if (cssStyle.hasFontStyle()) {
+      entry.hasItalic = true;
+      entry.italic = cssStyle.fontStyle == CssFontStyle::Italic;
+    }
+    self->inlineStyleStack.push_back(entry);
+    self->updateEffectiveInlineStyle();
+  } else if (matches(name, BOLD_TAGS, NUM_BOLD_TAGS)) {
    self->boldUntilDepth = std::min(self->boldUntilDepth, self->depth);
-    self->depth += 1;
-    return;
-  }
-
-  if (matches(name, ITALIC_TAGS, NUM_ITALIC_TAGS)) {
+    // Push inline style entry for bold tag
+    StyleStackEntry entry;
+    entry.depth = self->depth;  // Track depth for matching pop
+    entry.hasBold = true;
+    entry.bold = true;
+    if (cssStyle.hasFontStyle()) {
+      entry.hasItalic = true;
+      entry.italic = cssStyle.fontStyle == CssFontStyle::Italic;
+    }
+    if (cssStyle.hasTextDecoration()) {
+      entry.hasUnderline = true;
+      entry.underline = cssStyle.textDecoration == CssTextDecoration::Underline;
+    }
+    self->inlineStyleStack.push_back(entry);
+    self->updateEffectiveInlineStyle();
+  } else if (matches(name, ITALIC_TAGS, NUM_ITALIC_TAGS)) {
    self->italicUntilDepth = std::min(self->italicUntilDepth, self->depth);
-    self->depth += 1;
-    return;
+    // Push inline style entry for italic tag
+    StyleStackEntry entry;
+    entry.depth = self->depth;  // Track depth for matching pop
+    entry.hasItalic = true;
+    entry.italic = true;
+    if (cssStyle.hasFontWeight()) {
+      entry.hasBold = true;
+      entry.bold = cssStyle.fontWeight == CssFontWeight::Bold;
+    }
+    if (cssStyle.hasTextDecoration()) {
+      entry.hasUnderline = true;
+      entry.underline = cssStyle.textDecoration == CssTextDecoration::Underline;
+    }
+    self->inlineStyleStack.push_back(entry);
+    self->updateEffectiveInlineStyle();
+  } else if (strcmp(name, "span") == 0 || !isHeaderOrBlock(name)) {
+    // Handle span and other inline elements for CSS styling
+    if (cssStyle.hasFontWeight() || cssStyle.hasFontStyle() || cssStyle.hasTextDecoration()) {
+      StyleStackEntry entry;
+      entry.depth = self->depth;  // Track depth for matching pop
+      if (cssStyle.hasFontWeight()) {
+        entry.hasBold = true;
+        entry.bold = cssStyle.fontWeight == CssFontWeight::Bold;
+      }
+      if (cssStyle.hasFontStyle()) {
+        entry.hasItalic = true;
+        entry.italic = cssStyle.fontStyle == CssFontStyle::Italic;
+      }
+      if (cssStyle.hasTextDecoration()) {
+        entry.hasUnderline = true;
+        entry.underline = cssStyle.textDecoration == CssTextDecoration::Underline;
+      }
+      self->inlineStyleStack.push_back(entry);
+      self->updateEffectiveInlineStyle();
+    }
  }

  // Unprocessed tag, just increasing depth and continue forward
@@ -239,17 +366,27 @@ void XMLCALL ChapterHtmlSlimParser::characterData(void* userData, const XML_Char
 void XMLCALL ChapterHtmlSlimParser::endElement(void* userData, const XML_Char* name) {
  auto* self = static_cast<ChapterHtmlSlimParser*>(userData);

-  if (self->partWordBufferIndex > 0) {
-    // Only flush out part word buffer if we're closing a block tag or are at the top of the HTML file.
-    // We don't want to flush out content when closing inline tags like <span>.
-    // Currently this also flushes out on closing <b> and <i> tags, but they are line tags so that shouldn't happen,
-    // text styling needs to be overhauled to fix it.
-    const bool shouldBreakText =
-        matches(name, BLOCK_TAGS, NUM_BLOCK_TAGS) || matches(name, HEADER_TAGS, NUM_HEADER_TAGS) ||
-        matches(name, BOLD_TAGS, NUM_BOLD_TAGS) || matches(name, ITALIC_TAGS, NUM_ITALIC_TAGS) ||
-        strcmp(name, "table") == 0 || matches(name, IMAGE_TAGS, NUM_IMAGE_TAGS) || self->depth == 1;
+  // Check if any style state will change after we decrement depth
+  // If so, we MUST flush the partWordBuffer with the CURRENT style first
+  // Note: depth hasn't been decremented yet, so we check against (depth - 1)
+  const bool willPopStyleStack =
+      !self->inlineStyleStack.empty() && self->inlineStyleStack.back().depth == self->depth - 1;
+  const bool willClearBold = self->boldUntilDepth == self->depth - 1;
+  const bool willClearItalic = self->italicUntilDepth == self->depth - 1;
+  const bool willClearUnderline = self->underlineUntilDepth == self->depth - 1;

-    if (shouldBreakText) {
+  const bool styleWillChange = willPopStyleStack || willClearBold || willClearItalic || willClearUnderline;
+  const bool headerOrBlockTag = isHeaderOrBlock(name);
+
+  // Flush buffer with current style BEFORE any style changes
+  if (self->partWordBufferIndex > 0) {
+    // Flush if style will change OR if we're closing a block/structural element
+    const bool shouldFlush = styleWillChange || headerOrBlockTag || matches(name, BOLD_TAGS, NUM_BOLD_TAGS) ||
+                             matches(name, ITALIC_TAGS, NUM_ITALIC_TAGS) ||
+                             matches(name, UNDERLINE_TAGS, NUM_UNDERLINE_TAGS) || strcmp(name, "table") == 0 ||
+                             matches(name, IMAGE_TAGS, NUM_IMAGE_TAGS) || self->depth == 1;
+
+    if (shouldFlush) {
      self->flushPartWordBuffer();
    }
  }
@@ -261,19 +398,40 @@ void XMLCALL ChapterHtmlSlimParser::endElement(void* userData, const XML_Char* n
    self->skipUntilDepth = INT_MAX;
  }

-  // Leaving bold
+  // Leaving bold tag
  if (self->boldUntilDepth == self->depth) {
    self->boldUntilDepth = INT_MAX;
  }

-  // Leaving italic
+  // Leaving italic tag
  if (self->italicUntilDepth == self->depth) {
    self->italicUntilDepth = INT_MAX;
  }
+
+  // Leaving underline tag
+  if (self->underlineUntilDepth == self->depth) {
+    self->underlineUntilDepth = INT_MAX;
+  }
+
+  // Pop from inline style stack if we pushed an entry at this depth
+  // This handles all inline elements: b, i, u, span, etc.
+  if (!self->inlineStyleStack.empty() && self->inlineStyleStack.back().depth == self->depth) {
+    self->inlineStyleStack.pop_back();
+    self->updateEffectiveInlineStyle();
+  }
+
+  // Clear block style when leaving header or block elements
+  if (headerOrBlockTag) {
+    self->currentCssStyle.reset();
+    self->updateEffectiveInlineStyle();
+  }
 }

 bool ChapterHtmlSlimParser::parseAndBuildPages() {
-  startNewTextBlock((TextBlock::Style)this->paragraphAlignment);
+  auto paragraphAlignmentBlockStyle = BlockStyle();
+  paragraphAlignmentBlockStyle.textAlignDefined = true;
+  paragraphAlignmentBlockStyle.alignment = static_cast<CssTextAlign>(this->paragraphAlignment);
+  startNewTextBlock(paragraphAlignmentBlockStyle);

  const XML_Parser parser = XML_ParserCreate(nullptr);
  int done;
@@ -362,7 +520,9 @@ void ChapterHtmlSlimParser::addLineToPage(std::shared_ptr<TextBlock> line) {
    currentPageNextY = 0;
  }

-  currentPage->elements.push_back(std::make_shared<PageLine>(line, 0, currentPageNextY));
+  // Apply horizontal left inset (margin + padding) as x position offset
+  const int16_t xOffset = line->getBlockStyle().leftInset();
+  currentPage->elements.push_back(std::make_shared<PageLine>(line, xOffset, currentPageNextY));
  currentPageNextY += lineHeight;
 }

@@ -378,10 +538,34 @@ void ChapterHtmlSlimParser::makePages() {
  }

  const int lineHeight = renderer.getLineHeight(fontId) * lineCompression;
+
+  // Apply top spacing before the paragraph (stored in pixels)
+  const BlockStyle& blockStyle = currentTextBlock->getBlockStyle();
+  if (blockStyle.marginTop > 0) {
+    currentPageNextY += blockStyle.marginTop;
+  }
+  if (blockStyle.paddingTop > 0) {
+    currentPageNextY += blockStyle.paddingTop;
+  }
+
+  // Calculate effective width accounting for horizontal margins/padding
+  const int horizontalInset = blockStyle.totalHorizontalInset();
+  const uint16_t effectiveWidth =
+      (horizontalInset < viewportWidth) ? static_cast<uint16_t>(viewportWidth - horizontalInset) : viewportWidth;
+
  currentTextBlock->layoutAndExtractLines(
-      renderer, fontId, viewportWidth,
+      renderer, fontId, effectiveWidth,
      [this](const std::shared_ptr<TextBlock>& textBlock) { addLineToPage(textBlock); });
-  // Extra paragraph spacing if enabled
+
+  // Apply bottom spacing after the paragraph (stored in pixels)
+  if (blockStyle.marginBottom > 0) {
+    currentPageNextY += blockStyle.marginBottom;
+  }
+  if (blockStyle.paddingBottom > 0) {
+    currentPageNextY += blockStyle.paddingBottom;
+  }
+
+  // Extra paragraph spacing if enabled (default behavior)
  if (extraParagraphSpacing) {
    currentPageNextY += lineHeight / 2;
  }
@@ -8,6 +8,8 @@

 #include "../ParsedText.h"
 #include "../blocks/TextBlock.h"
+#include "../css/CssParser.h"
+#include "../css/CssStyle.h"

 class Page;
 class GfxRenderer;
@@ -23,6 +25,7 @@ class ChapterHtmlSlimParser {
  int skipUntilDepth = INT_MAX;
  int boldUntilDepth = INT_MAX;
  int italicUntilDepth = INT_MAX;
+  int underlineUntilDepth = INT_MAX;
  // buffer for building up words from characters, will auto break if longer than this
  // leave one char at end for null pointer
  char partWordBuffer[MAX_WORD_SIZE + 1] = {};
@@ -37,8 +40,23 @@ class ChapterHtmlSlimParser {
  uint16_t viewportWidth;
  uint16_t viewportHeight;
  bool hyphenationEnabled;
+  const CssParser* cssParser;

-  void startNewTextBlock(TextBlock::Style style);
+  // Style tracking (replaces depth-based approach)
+  struct StyleStackEntry {
+    int depth = 0;
+    bool hasBold = false, bold = false;
+    bool hasItalic = false, italic = false;
+    bool hasUnderline = false, underline = false;
+  };
+  std::vector<StyleStackEntry> inlineStyleStack;
+  CssStyle currentCssStyle;
+  bool effectiveBold = false;
+  bool effectiveItalic = false;
+  bool effectiveUnderline = false;
+
+  void updateEffectiveInlineStyle();
+  void startNewTextBlock(const BlockStyle& blockStyle);
  void flushPartWordBuffer();
  void makePages();
  // XML callbacks
@@ -52,7 +70,8 @@ class ChapterHtmlSlimParser {
                                 const uint8_t paragraphAlignment, const uint16_t viewportWidth,
                                 const uint16_t viewportHeight, const bool hyphenationEnabled,
                                 const std::function<void(std::unique_ptr<Page>)>& completePageFn,
-                                 const std::function<void()>& popupFn = nullptr)
+                                 const std::function<void()>& popupFn = nullptr, const CssParser* cssParser = nullptr)
+
      : filepath(filepath),
        renderer(renderer),
        fontId(fontId),
@@ -63,7 +82,9 @@ class ChapterHtmlSlimParser {
        viewportHeight(viewportHeight),
        hyphenationEnabled(hyphenationEnabled),
        completePageFn(completePageFn),
-        popupFn(popupFn) {}
+        popupFn(popupFn),
+        cssParser(cssParser) {}
+
  ~ChapterHtmlSlimParser() = default;
  bool parseAndBuildPages();
  void addLineToPage(std::shared_ptr<TextBlock> line);
@@ -8,6 +8,7 @@

 namespace {
 constexpr char MEDIA_TYPE_NCX[] = "application/x-dtbncx+xml";
+constexpr char MEDIA_TYPE_CSS[] = "text/css";
 constexpr char itemCacheFile[] = "/.items.bin";
 }  // namespace

@@ -218,6 +219,11 @@ void XMLCALL ContentOpfParser::startElement(void* userData, const XML_Char* name
      }
    }

+    // Collect CSS files
+    if (mediaType == MEDIA_TYPE_CSS) {
+      self->cssFiles.push_back(href);
+    }
+
    // EPUB 3: Check for nav document (properties contains "nav")
    if (!properties.empty() && self->tocNavPath.empty()) {
      // Properties is space-separated, check if "nav" is present as a word
@@ -64,6 +64,7 @@ class ContentOpfParser final : public Print {
  std::string tocNavPath;  // EPUB 3 nav document path
  std::string coverItemHref;
  std::string textReferenceHref;
+  std::vector<std::string> cssFiles;  // CSS stylesheet paths

  explicit ContentOpfParser(const std::string& cachePath, const std::string& baseContentPath, const size_t xmlSize,
                            BookMetadataCache* cache)