diff --git a/lib/Epub/Epub.cpp b/lib/Epub/Epub.cpp
index 87033f0f..095d0546 100644
--- a/lib/Epub/Epub.cpp
+++ b/lib/Epub/Epub.cpp
@@ -208,30 +208,14 @@ bool Epub::parseTocNavFile() const {
   return true;
 }
 
-std::string Epub::getCssRulesCache() const { return cachePath + "/css_rules.cache"; }
-
-bool Epub::loadCssRulesFromCache() const {
-  FsFile cssCacheFile;
-  if (Storage.openFileForRead("EBP", getCssRulesCache(), cssCacheFile)) {
-    if (cssParser->loadFromCache(cssCacheFile)) {
-      cssCacheFile.close();
-      LOG_DBG("EBP", "Loaded CSS rules from cache");
-      return true;
-    }
-    cssCacheFile.close();
-    LOG_DBG("EBP", "CSS cache invalid, reparsing");
-  }
-  return false;
-}
-
 void Epub::parseCssFiles() const {
   if (cssFiles.empty()) {
     LOG_DBG("EBP", "No CSS files to parse, but CssParser created for inline styles");
   }
 
-  // Try to load from CSS cache first
-  if (!loadCssRulesFromCache()) {
-    // Cache miss - parse CSS files
+  // See if we have a cached version of the CSS rules
+  if (!cssParser->hasCache()) {
+    // No cache yet - parse CSS files
     for (const auto& cssPath : cssFiles) {
       LOG_DBG("EBP", "Parsing CSS file: %s", cssPath.c_str());
 
@@ -262,11 +246,10 @@ void Epub::parseCssFiles() const {
     }
 
     // Save to cache for next time
-    FsFile cssCacheFile;
-    if (Storage.openFileForWrite("EBP", getCssRulesCache(), cssCacheFile)) {
-      cssParser->saveToCache(cssCacheFile);
-      cssCacheFile.close();
+    if (!cssParser->saveToCache()) {
+      LOG_ERR("EBP", "Failed to save CSS rules to cache");
     }
+    cssParser->clear();
 
     LOG_DBG("EBP", "Loaded %zu CSS style rules from %zu files", cssParser->ruleCount(), cssFiles.size());
   }
@@ -279,11 +262,11 @@ bool Epub::load(const bool buildIfMissing, const bool skipLoadingCss) {
   // Initialize spine/TOC cache
   bookMetadataCache.reset(new BookMetadataCache(cachePath));
   // Always create CssParser - needed for inline style parsing even without CSS files
-  cssParser.reset(new CssParser());
+  cssParser.reset(new CssParser(cachePath));
 
   // Try to load existing cache first
   if (bookMetadataCache->load()) {
-    if (!skipLoadingCss && !loadCssRulesFromCache()) {
+    if (!skipLoadingCss && !cssParser->hasCache()) {
       LOG_DBG("EBP", "Warning: CSS rules cache not found, attempting to parse CSS files");
       // to get CSS file list
       if (!parseContentOpf(bookMetadataCache->coreMetadata)) {
diff --git a/lib/Epub/Epub.h b/lib/Epub/Epub.h
index 0efde410..cde9d210 100644
--- a/lib/Epub/Epub.h
+++ b/lib/Epub/Epub.h
@@ -35,8 +35,6 @@ class Epub {
   bool parseTocNcxFile() const;
   bool parseTocNavFile() const;
   void parseCssFiles() const;
-  std::string getCssRulesCache() const;
-  bool loadCssRulesFromCache() const;
 
  public:
   explicit Epub(std::string filepath, const std::string& cacheDir) : filepath(std::move(filepath)) {
@@ -73,5 +71,5 @@ class Epub {
 
   size_t getBookSize() const;
   float calculateProgress(int currentSpineIndex, float currentSpineRead) const;
-  const CssParser* getCssParser() const { return cssParser.get(); }
+  CssParser* getCssParser() const { return cssParser.get(); }
 };
diff --git a/lib/Epub/Epub/Section.cpp b/lib/Epub/Epub/Section.cpp
index 5039c57b..496bdb5c 100644
--- a/lib/Epub/Epub/Section.cpp
+++ b/lib/Epub/Epub/Section.cpp
@@ -181,11 +181,20 @@ bool Section::createSectionFile(const int fontId, const float lineCompression, c
                          viewportHeight, hyphenationEnabled, embeddedStyle);
   std::vector<uint32_t> lut = {};
 
+  CssParser* cssParser = nullptr;
+  if (embeddedStyle) {
+    cssParser = epub->getCssParser();
+    if (cssParser) {
+      if (!cssParser->loadFromCache()) {
+        LOG_ERR("SCT", "Failed to load CSS from cache");
+      }
+    }
+  }
   ChapterHtmlSlimParser visitor(
       tmpHtmlPath, renderer, fontId, lineCompression, extraParagraphSpacing, paragraphAlignment, viewportWidth,
       viewportHeight, hyphenationEnabled,
       [this, &lut](std::unique_ptr<Page> page) { lut.emplace_back(this->onPageComplete(std::move(page))); },
-      embeddedStyle, popupFn, embeddedStyle ? epub->getCssParser() : nullptr);
+      embeddedStyle, popupFn, cssParser);
   Hyphenator::setPreferredLanguage(epub->getLanguage());
   success = visitor.parseAndBuildPages();
 
@@ -194,6 +203,9 @@ bool Section::createSectionFile(const int fontId, const float lineCompression, c
     LOG_ERR("SCT", "Failed to parse XML and build pages");
     file.close();
     Storage.remove(filePath.c_str());
+    if (cssParser) {
+      cssParser->clear();
+    }
     return false;
   }
 
@@ -220,6 +232,9 @@ bool Section::createSectionFile(const int fontId, const float lineCompression, c
   serialization::writePod(file, pageCount);
   serialization::writePod(file, lutOffset);
   file.close();
+  if (cssParser) {
+    cssParser->clear();
+  }
   return true;
 }
 
diff --git a/lib/Epub/Epub/css/CssParser.cpp b/lib/Epub/Epub/css/CssParser.cpp
index 0a11e80e..590b2215 100644
--- a/lib/Epub/Epub/css/CssParser.cpp
+++ b/lib/Epub/Epub/css/CssParser.cpp
@@ -1,144 +1,57 @@
 #include "CssParser.h"
 
+#include <Arduino.h>
 #include <Logging.h>
 
 #include <algorithm>
+#include <array>
 #include <cctype>
+#include <string_view>
 
 namespace {
 
+// Stack-allocated string buffer to avoid heap reallocations during parsing
+// Provides string-like interface with fixed capacity
+struct StackBuffer {
+  static constexpr size_t CAPACITY = 1024;
+  char data[CAPACITY];
+  size_t len = 0;
+
+  void push_back(char c) {
+    if (len < CAPACITY - 1) {
+      data[len++] = c;
+    }
+  }
+
+  void clear() { len = 0; }
+  bool empty() const { return len == 0; }
+  size_t size() const { return len; }
+
+  // Get string view of current content (zero-copy)
+  std::string_view view() const { return std::string_view(data, len); }
+
+  // Convert to string for passing to functions (single allocation)
+  std::string str() const { return std::string(data, len); }
+};
+
 // Buffer size for reading CSS files
 constexpr size_t READ_BUFFER_SIZE = 512;
 
-// Maximum CSS file size we'll process (prevent memory issues)
-constexpr size_t MAX_CSS_SIZE = 64 * 1024;
+// Maximum number of CSS rules to store in the selector map
+// Prevents unbounded memory growth from pathological CSS files
+constexpr size_t MAX_RULES = 1500;
+
+// Minimum free heap required to apply CSS during rendering
+// If below this threshold, we skip CSS to avoid display artifacts.
+constexpr size_t MIN_FREE_HEAP_FOR_CSS = 48 * 1024;
+
+// Maximum length for a single selector string
+// Prevents parsing of extremely long or malformed selectors
+constexpr size_t MAX_SELECTOR_LENGTH = 256;
 
 // Check if character is CSS whitespace
 bool isCssWhitespace(const char c) { return c == ' ' || c == '\t' || c == '\n' || c == '\r' || c == '\f'; }
 
-// Read entire file into string (with size limit)
-std::string readFileContent(FsFile& file) {
-  std::string content;
-  content.reserve(std::min(static_cast<size_t>(file.size()), MAX_CSS_SIZE));
-
-  char buffer[READ_BUFFER_SIZE];
-  while (file.available() && content.size() < MAX_CSS_SIZE) {
-    const int bytesRead = file.read(buffer, sizeof(buffer));
-    if (bytesRead <= 0) break;
-    content.append(buffer, bytesRead);
-  }
-  return content;
-}
-
-// Remove CSS comments (/* ... */) from content
-std::string stripComments(const std::string& css) {
-  std::string result;
-  result.reserve(css.size());
-
-  size_t pos = 0;
-  while (pos < css.size()) {
-    // Look for start of comment
-    if (pos + 1 < css.size() && css[pos] == '/' && css[pos + 1] == '*') {
-      // Find end of comment
-      const size_t endPos = css.find("*/", pos + 2);
-      if (endPos == std::string::npos) {
-        // Unterminated comment - skip rest of file
-        break;
-      }
-      pos = endPos + 2;
-    } else {
-      result.push_back(css[pos]);
-      ++pos;
-    }
-  }
-  return result;
-}
-
-// Skip @-rules (like @media, @import, @font-face)
-// Returns position after the @-rule
-size_t skipAtRule(const std::string& css, const size_t start) {
-  // Find the end - either semicolon (simple @-rule) or matching brace
-  size_t pos = start + 1;  // Skip the '@'
-
-  // Skip identifier
-  while (pos < css.size() && (std::isalnum(css[pos]) || css[pos] == '-')) {
-    ++pos;
-  }
-
-  // Look for { or ;
-  int braceDepth = 0;
-  while (pos < css.size()) {
-    const char c = css[pos];
-    if (c == '{') {
-      ++braceDepth;
-    } else if (c == '}') {
-      --braceDepth;
-      if (braceDepth == 0) {
-        return pos + 1;
-      }
-    } else if (c == ';' && braceDepth == 0) {
-      return pos + 1;
-    }
-    ++pos;
-  }
-  return css.size();
-}
-
-// Extract next rule from CSS content
-// Returns true if a rule was found, with selector and body filled
-bool extractNextRule(const std::string& css, size_t& pos, std::string& selector, std::string& body) {
-  selector.clear();
-  body.clear();
-
-  // Skip whitespace and @-rules until we find a regular rule
-  while (pos < css.size()) {
-    // Skip whitespace
-    while (pos < css.size() && isCssWhitespace(css[pos])) {
-      ++pos;
-    }
-
-    if (pos >= css.size()) return false;
-
-    // Handle @-rules iteratively (avoids recursion/stack overflow)
-    if (css[pos] == '@') {
-      pos = skipAtRule(css, pos);
-      continue;  // Try again after skipping the @-rule
-    }
-
-    break;  // Found start of a regular rule
-  }
-
-  if (pos >= css.size()) return false;
-
-  // Find opening brace
-  const size_t bracePos = css.find('{', pos);
-  if (bracePos == std::string::npos) return false;
-
-  // Extract selector (everything before the brace)
-  selector = css.substr(pos, bracePos - pos);
-
-  // Find matching closing brace
-  int depth = 1;
-  const size_t bodyStart = bracePos + 1;
-  size_t bodyEnd = bodyStart;
-
-  while (bodyEnd < css.size() && depth > 0) {
-    if (css[bodyEnd] == '{')
-      ++depth;
-    else if (css[bodyEnd] == '}')
-      --depth;
-    ++bodyEnd;
-  }
-
-  // Extract body (between braces)
-  if (bodyEnd > bodyStart) {
-    body = css.substr(bodyStart, bodyEnd - bodyStart - 1);
-  }
-
-  pos = bodyEnd;
-  return true;
-}
-
 }  // anonymous namespace
 
 // String utilities implementation
@@ -167,6 +80,28 @@ std::string CssParser::normalized(const std::string& s) {
   return result;
 }
 
+void CssParser::normalizedInto(const std::string& s, std::string& out) {
+  out.clear();
+  out.reserve(s.size());
+
+  bool inSpace = true;  // Start true to skip leading space
+  for (const char c : s) {
+    if (isCssWhitespace(c)) {
+      if (!inSpace) {
+        out.push_back(' ');
+        inSpace = true;
+      }
+    } else {
+      out.push_back(static_cast<char>(std::tolower(static_cast<unsigned char>(c))));
+      inSpace = false;
+    }
+  }
+
+  if (!out.empty() && out.back() == ' ') {
+    out.pop_back();
+  }
+}
+
 std::vector<std::string> CssParser::splitOnChar(const std::string& s, const char delimiter) {
   std::vector<std::string> parts;
   size_t start = 0;
@@ -290,129 +225,95 @@ CssLength CssParser::interpretLength(const std::string& val) {
 
   return CssLength{numericValue, unit};
 }
+// Declaration parsing
 
-int8_t CssParser::interpretSpacing(const std::string& val) {
-  const std::string v = normalized(val);
-  if (v.empty()) return 0;
+void CssParser::parseDeclarationIntoStyle(const std::string& decl, CssStyle& style, std::string& propNameBuf,
+                                          std::string& propValueBuf) {
+  const size_t colonPos = decl.find(':');
+  if (colonPos == std::string::npos || colonPos == 0) return;
 
-  // For spacing, we convert to "lines" (discrete units for e-ink)
-  // 1em ≈ 1 line, percentages based on ~30 lines per page
+  normalizedInto(decl.substr(0, colonPos), propNameBuf);
+  normalizedInto(decl.substr(colonPos + 1), propValueBuf);
 
-  float multiplier = 0.0f;
-  size_t unitStart = v.size();
+  if (propNameBuf.empty() || propValueBuf.empty()) return;
 
-  for (size_t i = 0; i < v.size(); ++i) {
-    const char c = v[i];
-    if (!std::isdigit(c) && c != '.' && c != '-' && c != '+') {
-      unitStart = i;
-      break;
+  if (propNameBuf == "text-align") {
+    style.textAlign = interpretAlignment(propValueBuf);
+    style.defined.textAlign = 1;
+  } else if (propNameBuf == "font-style") {
+    style.fontStyle = interpretFontStyle(propValueBuf);
+    style.defined.fontStyle = 1;
+  } else if (propNameBuf == "font-weight") {
+    style.fontWeight = interpretFontWeight(propValueBuf);
+    style.defined.fontWeight = 1;
+  } else if (propNameBuf == "text-decoration" || propNameBuf == "text-decoration-line") {
+    style.textDecoration = interpretDecoration(propValueBuf);
+    style.defined.textDecoration = 1;
+  } else if (propNameBuf == "text-indent") {
+    style.textIndent = interpretLength(propValueBuf);
+    style.defined.textIndent = 1;
+  } else if (propNameBuf == "margin-top") {
+    style.marginTop = interpretLength(propValueBuf);
+    style.defined.marginTop = 1;
+  } else if (propNameBuf == "margin-bottom") {
+    style.marginBottom = interpretLength(propValueBuf);
+    style.defined.marginBottom = 1;
+  } else if (propNameBuf == "margin-left") {
+    style.marginLeft = interpretLength(propValueBuf);
+    style.defined.marginLeft = 1;
+  } else if (propNameBuf == "margin-right") {
+    style.marginRight = interpretLength(propValueBuf);
+    style.defined.marginRight = 1;
+  } else if (propNameBuf == "margin") {
+    const auto values = splitWhitespace(propValueBuf);
+    if (!values.empty()) {
+      style.marginTop = interpretLength(values[0]);
+      style.marginRight = values.size() >= 2 ? interpretLength(values[1]) : style.marginTop;
+      style.marginBottom = values.size() >= 3 ? interpretLength(values[2]) : style.marginTop;
+      style.marginLeft = values.size() >= 4 ? interpretLength(values[3]) : style.marginRight;
+      style.defined.marginTop = style.defined.marginRight = style.defined.marginBottom = style.defined.marginLeft = 1;
+    }
+  } else if (propNameBuf == "padding-top") {
+    style.paddingTop = interpretLength(propValueBuf);
+    style.defined.paddingTop = 1;
+  } else if (propNameBuf == "padding-bottom") {
+    style.paddingBottom = interpretLength(propValueBuf);
+    style.defined.paddingBottom = 1;
+  } else if (propNameBuf == "padding-left") {
+    style.paddingLeft = interpretLength(propValueBuf);
+    style.defined.paddingLeft = 1;
+  } else if (propNameBuf == "padding-right") {
+    style.paddingRight = interpretLength(propValueBuf);
+    style.defined.paddingRight = 1;
+  } else if (propNameBuf == "padding") {
+    const auto values = splitWhitespace(propValueBuf);
+    if (!values.empty()) {
+      style.paddingTop = interpretLength(values[0]);
+      style.paddingRight = values.size() >= 2 ? interpretLength(values[1]) : style.paddingTop;
+      style.paddingBottom = values.size() >= 3 ? interpretLength(values[2]) : style.paddingTop;
+      style.paddingLeft = values.size() >= 4 ? interpretLength(values[3]) : style.paddingRight;
+      style.defined.paddingTop = style.defined.paddingRight = style.defined.paddingBottom = style.defined.paddingLeft =
+          1;
     }
   }
-
-  const std::string numPart = v.substr(0, unitStart);
-  const std::string unitPart = v.substr(unitStart);
-
-  if (unitPart == "em" || unitPart == "rem") {
-    multiplier = 1.0f;  // 1em = 1 line
-  } else if (unitPart == "%") {
-    multiplier = 0.3f;  // ~30 lines per page, so 10% = 3 lines
-  } else {
-    return 0;  // Unsupported unit for spacing
-  }
-
-  char* endPtr = nullptr;
-  const float numericValue = std::strtof(numPart.c_str(), &endPtr);
-
-  if (endPtr == numPart.c_str()) return 0;
-
-  int lines = static_cast<int>(numericValue * multiplier);
-
-  // Clamp to reasonable range (0-2 lines)
-  if (lines < 0) lines = 0;
-  if (lines > 2) lines = 2;
-
-  return static_cast<int8_t>(lines);
 }
 
-// Declaration parsing
-
 CssStyle CssParser::parseDeclarations(const std::string& declBlock) {
   CssStyle style;
+  std::string propNameBuf;
+  std::string propValueBuf;
 
-  // Split declarations by semicolon
-  const auto declarations = splitOnChar(declBlock, ';');
-
-  for (const auto& decl : declarations) {
-    // Find colon separator
-    const size_t colonPos = decl.find(':');
-    if (colonPos == std::string::npos || colonPos == 0) continue;
-
-    std::string propName = normalized(decl.substr(0, colonPos));
-    std::string propValue = normalized(decl.substr(colonPos + 1));
-
-    if (propName.empty() || propValue.empty()) continue;
-
-    // Match property and set value
-    if (propName == "text-align") {
-      style.textAlign = interpretAlignment(propValue);
-      style.defined.textAlign = 1;
-    } else if (propName == "font-style") {
-      style.fontStyle = interpretFontStyle(propValue);
-      style.defined.fontStyle = 1;
-    } else if (propName == "font-weight") {
-      style.fontWeight = interpretFontWeight(propValue);
-      style.defined.fontWeight = 1;
-    } else if (propName == "text-decoration" || propName == "text-decoration-line") {
-      style.textDecoration = interpretDecoration(propValue);
-      style.defined.textDecoration = 1;
-    } else if (propName == "text-indent") {
-      style.textIndent = interpretLength(propValue);
-      style.defined.textIndent = 1;
-    } else if (propName == "margin-top") {
-      style.marginTop = interpretLength(propValue);
-      style.defined.marginTop = 1;
-    } else if (propName == "margin-bottom") {
-      style.marginBottom = interpretLength(propValue);
-      style.defined.marginBottom = 1;
-    } else if (propName == "margin-left") {
-      style.marginLeft = interpretLength(propValue);
-      style.defined.marginLeft = 1;
-    } else if (propName == "margin-right") {
-      style.marginRight = interpretLength(propValue);
-      style.defined.marginRight = 1;
-    } else if (propName == "margin") {
-      // Shorthand: 1-4 values for top, right, bottom, left
-      const auto values = splitWhitespace(propValue);
-      if (!values.empty()) {
-        style.marginTop = interpretLength(values[0]);
-        style.marginRight = values.size() >= 2 ? interpretLength(values[1]) : style.marginTop;
-        style.marginBottom = values.size() >= 3 ? interpretLength(values[2]) : style.marginTop;
-        style.marginLeft = values.size() >= 4 ? interpretLength(values[3]) : style.marginRight;
-        style.defined.marginTop = style.defined.marginRight = style.defined.marginBottom = style.defined.marginLeft = 1;
-      }
-    } else if (propName == "padding-top") {
-      style.paddingTop = interpretLength(propValue);
-      style.defined.paddingTop = 1;
-    } else if (propName == "padding-bottom") {
-      style.paddingBottom = interpretLength(propValue);
-      style.defined.paddingBottom = 1;
-    } else if (propName == "padding-left") {
-      style.paddingLeft = interpretLength(propValue);
-      style.defined.paddingLeft = 1;
-    } else if (propName == "padding-right") {
-      style.paddingRight = interpretLength(propValue);
-      style.defined.paddingRight = 1;
-    } else if (propName == "padding") {
-      // Shorthand: 1-4 values for top, right, bottom, left
-      const auto values = splitWhitespace(propValue);
-      if (!values.empty()) {
-        style.paddingTop = interpretLength(values[0]);
-        style.paddingRight = values.size() >= 2 ? interpretLength(values[1]) : style.paddingTop;
-        style.paddingBottom = values.size() >= 3 ? interpretLength(values[2]) : style.paddingTop;
-        style.paddingLeft = values.size() >= 4 ? interpretLength(values[3]) : style.paddingRight;
-        style.defined.paddingTop = style.defined.paddingRight = style.defined.paddingBottom =
-            style.defined.paddingLeft = 1;
+  size_t start = 0;
+  for (size_t i = 0; i <= declBlock.size(); ++i) {
+    if (i == declBlock.size() || declBlock[i] == ';') {
+      if (i > start) {
+        const size_t len = i - start;
+        std::string decl = declBlock.substr(start, len);
+        if (!decl.empty()) {
+          parseDeclarationIntoStyle(decl, style, propNameBuf, propValueBuf);
+        }
       }
+      start = i + 1;
     }
   }
 
@@ -421,20 +322,33 @@ CssStyle CssParser::parseDeclarations(const std::string& declBlock) {
 
 // Rule processing
 
-void CssParser::processRuleBlock(const std::string& selectorGroup, const std::string& declarations) {
-  const CssStyle style = parseDeclarations(declarations);
-
-  // Only store if any properties were set
-  if (!style.defined.anySet()) return;
+void CssParser::processRuleBlockWithStyle(const std::string& selectorGroup, const CssStyle& style) {
+  // Check if we've reached the rule limit before processing
+  if (rulesBySelector_.size() >= MAX_RULES) {
+    LOG_DBG("CSS", "Reached max rules limit (%zu), stopping CSS parsing", MAX_RULES);
+    return;
+  }
 
   // Handle comma-separated selectors
   const auto selectors = splitOnChar(selectorGroup, ',');
 
   for (const auto& sel : selectors) {
+    // Validate selector length before processing
+    if (sel.size() > MAX_SELECTOR_LENGTH) {
+      LOG_DBG("CSS", "Selector too long (%zu > %zu), skipping", sel.size(), MAX_SELECTOR_LENGTH);
+      continue;
+    }
+
     // Normalize the selector
     std::string key = normalized(sel);
     if (key.empty()) continue;
 
+    // Skip if this would exceed the rule limit
+    if (rulesBySelector_.size() >= MAX_RULES) {
+      LOG_DBG("CSS", "Reached max rules limit, stopping selector processing");
+      return;
+    }
+
     // Store or merge with existing
     auto it = rulesBySelector_.find(key);
     if (it != rulesBySelector_.end()) {
@@ -453,30 +367,158 @@ bool CssParser::loadFromStream(FsFile& source) {
     return false;
   }
 
-  // Read file content
-  const std::string content = readFileContent(source);
-  if (content.empty()) {
-    return true;  // Empty file is valid
+  size_t totalRead = 0;
+
+  // Use stack-allocated buffers for parsing to avoid heap reallocations
+  StackBuffer selector;
+  StackBuffer declBuffer;
+  // Keep these as std::string since they're passed by reference to parseDeclarationIntoStyle
+  std::string propNameBuf;
+  std::string propValueBuf;
+
+  bool inComment = false;
+  bool maybeSlash = false;
+  bool prevStar = false;
+
+  bool inAtRule = false;
+  int atDepth = 0;
+
+  int bodyDepth = 0;
+  bool skippingRule = false;
+  CssStyle currentStyle;
+
+  auto handleChar = [&](const char c) {
+    if (inAtRule) {
+      if (c == '{') {
+        ++atDepth;
+      } else if (c == '}') {
+        if (atDepth > 0) --atDepth;
+        if (atDepth == 0) inAtRule = false;
+      } else if (c == ';' && atDepth == 0) {
+        inAtRule = false;
+      }
+      return;
+    }
+
+    if (bodyDepth == 0) {
+      if (selector.empty() && isCssWhitespace(c)) {
+        return;
+      }
+      if (c == '@' && selector.empty()) {
+        inAtRule = true;
+        atDepth = 0;
+        return;
+      }
+      if (c == '{') {
+        bodyDepth = 1;
+        currentStyle = CssStyle{};
+        declBuffer.clear();
+        if (selector.size() > MAX_SELECTOR_LENGTH * 4) {
+          skippingRule = true;
+        }
+        return;
+      }
+      selector.push_back(c);
+      return;
+    }
+
+    // bodyDepth > 0
+    if (c == '{') {
+      ++bodyDepth;
+      return;
+    }
+    if (c == '}') {
+      --bodyDepth;
+      if (bodyDepth == 0) {
+        if (!skippingRule && !declBuffer.empty()) {
+          parseDeclarationIntoStyle(declBuffer.str(), currentStyle, propNameBuf, propValueBuf);
+        }
+        if (!skippingRule) {
+          processRuleBlockWithStyle(selector.str(), currentStyle);
+        }
+        selector.clear();
+        declBuffer.clear();
+        skippingRule = false;
+        return;
+      }
+      return;
+    }
+    if (bodyDepth > 1) {
+      return;
+    }
+    if (!skippingRule) {
+      if (c == ';') {
+        if (!declBuffer.empty()) {
+          parseDeclarationIntoStyle(declBuffer.str(), currentStyle, propNameBuf, propValueBuf);
+          declBuffer.clear();
+        }
+      } else {
+        declBuffer.push_back(c);
+      }
+    }
+  };
+
+  char buffer[READ_BUFFER_SIZE];
+  while (source.available()) {
+    int bytesRead = source.read(buffer, sizeof(buffer));
+    if (bytesRead <= 0) break;
+
+    totalRead += static_cast<size_t>(bytesRead);
+
+    for (int i = 0; i < bytesRead; ++i) {
+      const char c = buffer[i];
+
+      if (inComment) {
+        if (prevStar && c == '/') {
+          inComment = false;
+          prevStar = false;
+          continue;
+        }
+        prevStar = c == '*';
+        continue;
+      }
+
+      if (maybeSlash) {
+        if (c == '*') {
+          inComment = true;
+          maybeSlash = false;
+          prevStar = false;
+          continue;
+        }
+        handleChar('/');
+        maybeSlash = false;
+        // fall through to process current char
+      }
+
+      if (c == '/') {
+        maybeSlash = true;
+        continue;
+      }
+
+      handleChar(c);
+    }
   }
 
-  // Remove comments
-  const std::string cleaned = stripComments(content);
-
-  // Parse rules
-  size_t pos = 0;
-  std::string selector, body;
-
-  while (extractNextRule(cleaned, pos, selector, body)) {
-    processRuleBlock(selector, body);
+  if (maybeSlash) {
+    handleChar('/');
   }
 
-  LOG_DBG("CSS", "Parsed %zu rules", rulesBySelector_.size());
+  LOG_DBG("CSS", "Parsed %zu rules from %zu bytes", rulesBySelector_.size(), totalRead);
   return true;
 }
 
 // Style resolution
 
 CssStyle CssParser::resolveStyle(const std::string& tagName, const std::string& classAttr) const {
+  static bool lowHeapWarningLogged = false;
+  if (ESP.getFreeHeap() < MIN_FREE_HEAP_FOR_CSS) {
+    if (!lowHeapWarningLogged) {
+      lowHeapWarningLogged = true;
+      LOG_DBG("CSS", "Warning: low heap (%u bytes) below MIN_FREE_HEAP_FOR_CSS (%u), returning empty style",
+              ESP.getFreeHeap(), static_cast<unsigned>(MIN_FREE_HEAP_FOR_CSS));
+    }
+    return CssStyle{};
+  }
   CssStyle result;
   const std::string tag = normalized(tagName);
 
@@ -521,9 +563,17 @@ CssStyle CssParser::parseInlineStyle(const std::string& styleValue) { return par
 
 // Cache format version - increment when format changes
 constexpr uint8_t CSS_CACHE_VERSION = 2;
+constexpr char rulesCache[] = "/css_rules.cache";
 
-bool CssParser::saveToCache(FsFile& file) const {
-  if (!file) {
+bool CssParser::hasCache() const { return Storage.exists((cachePath + rulesCache).c_str()); }
+
+bool CssParser::saveToCache() const {
+  if (cachePath.empty()) {
+    return false;
+  }
+
+  FsFile file;
+  if (!Storage.openFileForWrite("CSS", cachePath + rulesCache, file)) {
     return false;
   }
 
@@ -583,11 +633,17 @@ bool CssParser::saveToCache(FsFile& file) const {
   }
 
   LOG_DBG("CSS", "Saved %u rules to cache", ruleCount);
+  file.close();
   return true;
 }
 
-bool CssParser::loadFromCache(FsFile& file) {
-  if (!file) {
+bool CssParser::loadFromCache() {
+  if (cachePath.empty()) {
+    return false;
+  }
+
+  FsFile file;
+  if (!Storage.openFileForRead("CSS", cachePath + rulesCache, file)) {
     return false;
   }
 
@@ -598,12 +654,14 @@ bool CssParser::loadFromCache(FsFile& file) {
   uint8_t version = 0;
   if (file.read(&version, 1) != 1 || version != CSS_CACHE_VERSION) {
     LOG_DBG("CSS", "Cache version mismatch (got %u, expected %u)", version, CSS_CACHE_VERSION);
+    file.close();
     return false;
   }
 
   // Read rule count
   uint16_t ruleCount = 0;
   if (file.read(&ruleCount, sizeof(ruleCount)) != sizeof(ruleCount)) {
+    file.close();
     return false;
   }
 
@@ -613,6 +671,7 @@ bool CssParser::loadFromCache(FsFile& file) {
     uint16_t selectorLen = 0;
     if (file.read(&selectorLen, sizeof(selectorLen)) != sizeof(selectorLen)) {
       rulesBySelector_.clear();
+      file.close();
       return false;
     }
 
@@ -620,6 +679,7 @@ bool CssParser::loadFromCache(FsFile& file) {
     selector.resize(selectorLen);
     if (file.read(&selector[0], selectorLen) != selectorLen) {
       rulesBySelector_.clear();
+      file.close();
       return false;
     }
 
@@ -629,24 +689,28 @@ bool CssParser::loadFromCache(FsFile& file) {
 
     if (file.read(&enumVal, 1) != 1) {
       rulesBySelector_.clear();
+      file.close();
       return false;
     }
     style.textAlign = static_cast<CssTextAlign>(enumVal);
 
     if (file.read(&enumVal, 1) != 1) {
       rulesBySelector_.clear();
+      file.close();
       return false;
     }
     style.fontStyle = static_cast<CssFontStyle>(enumVal);
 
     if (file.read(&enumVal, 1) != 1) {
       rulesBySelector_.clear();
+      file.close();
       return false;
     }
     style.fontWeight = static_cast<CssFontWeight>(enumVal);
 
     if (file.read(&enumVal, 1) != 1) {
       rulesBySelector_.clear();
+      file.close();
       return false;
     }
     style.textDecoration = static_cast<CssTextDecoration>(enumVal);
@@ -668,6 +732,7 @@ bool CssParser::loadFromCache(FsFile& file) {
         !readLength(style.marginLeft) || !readLength(style.marginRight) || !readLength(style.paddingTop) ||
         !readLength(style.paddingBottom) || !readLength(style.paddingLeft) || !readLength(style.paddingRight)) {
       rulesBySelector_.clear();
+      file.close();
       return false;
     }
 
@@ -675,6 +740,7 @@ bool CssParser::loadFromCache(FsFile& file) {
     uint16_t definedBits = 0;
     if (file.read(&definedBits, sizeof(definedBits)) != sizeof(definedBits)) {
       rulesBySelector_.clear();
+      file.close();
       return false;
     }
     style.defined.textAlign = (definedBits & 1 << 0) != 0;
@@ -695,5 +761,6 @@ bool CssParser::loadFromCache(FsFile& file) {
   }
 
   LOG_DBG("CSS", "Loaded %u rules from cache", ruleCount);
+  file.close();
   return true;
 }
diff --git a/lib/Epub/Epub/css/CssParser.h b/lib/Epub/Epub/css/CssParser.h
index 5605553d..60f70d23 100644
--- a/lib/Epub/Epub/css/CssParser.h
+++ b/lib/Epub/Epub/css/CssParser.h
@@ -4,6 +4,7 @@
 
 #include <string>
 #include <unordered_map>
+#include <utility>
 #include <vector>
 
 #include "CssStyle.h"
@@ -29,7 +30,7 @@
  */
 class CssParser {
  public:
-  CssParser() = default;
+  explicit CssParser(std::string cachePath) : cachePath(std::move(cachePath)) {}
   ~CssParser() = default;
 
   // Non-copyable
@@ -76,28 +77,35 @@ class CssParser {
    */
   void clear() { rulesBySelector_.clear(); }
 
+  /**
+   * Check if CSS rules cache file exists
+   */
+  bool hasCache() const;
+
   /**
    * Save parsed CSS rules to a cache file.
-   * @param file Open file handle to write to
    * @return true if cache was written successfully
    */
-  bool saveToCache(FsFile& file) const;
+  bool saveToCache() const;
 
   /**
    * Load CSS rules from a cache file.
    * Clears any existing rules before loading.
-   * @param file Open file handle to read from
    * @return true if cache was loaded successfully
    */
-  bool loadFromCache(FsFile& file);
+  bool loadFromCache();
 
  private:
   // Storage: maps normalized selector -> style properties
   std::unordered_map<std::string, CssStyle> rulesBySelector_;
 
+  std::string cachePath;
+
   // Internal parsing helpers
-  void processRuleBlock(const std::string& selectorGroup, const std::string& declarations);
+  void processRuleBlockWithStyle(const std::string& selectorGroup, const CssStyle& style);
   static CssStyle parseDeclarations(const std::string& declBlock);
+  static void parseDeclarationIntoStyle(const std::string& decl, CssStyle& style, std::string& propNameBuf,
+                                        std::string& propValueBuf);
 
   // Individual property value parsers
   static CssTextAlign interpretAlignment(const std::string& val);
@@ -105,10 +113,10 @@ class CssParser {
   static CssFontWeight interpretFontWeight(const std::string& val);
   static CssTextDecoration interpretDecoration(const std::string& val);
   static CssLength interpretLength(const std::string& val);
-  static int8_t interpretSpacing(const std::string& val);
 
   // String utilities
   static std::string normalized(const std::string& s);
+  static void normalizedInto(const std::string& s, std::string& out);
   static std::vector<std::string> splitOnChar(const std::string& s, char delimiter);
   static std::vector<std::string> splitWhitespace(const std::string& s);
 };
diff --git a/platformio.ini b/platformio.ini
index 281eab17..20383f4f 100644
--- a/platformio.ini
+++ b/platformio.ini
@@ -27,10 +27,13 @@ build_flags =
 # https://libexpat.github.io/doc/api/latest/#XML_GE
   -DXML_GE=0
   -DXML_CONTEXT_BYTES=1024
-  -std=c++2a
+  -std=gnu++2a
 # Enable UTF-8 long file names in SdFat
   -DUSE_UTF8_LONG_NAMES=1
 
+build_unflags =
+  -std=gnu++11
+
 ; Board configuration
 board_build.flash_mode = dio
 board_build.flash_size = 16MB
diff --git a/src/RecentBooksStore.cpp b/src/RecentBooksStore.cpp
index 25092d85..1306c217 100644
--- a/src/RecentBooksStore.cpp
+++ b/src/RecentBooksStore.cpp
@@ -88,7 +88,7 @@ RecentBook RecentBooksStore::getDataFromBook(std::string path) const {
   // If epub, try to load the metadata for title/author and cover
   if (StringUtils::checkFileExtension(lastBookFileName, ".epub")) {
     Epub epub(path, "/.crosspoint");
-    epub.load(false);
+    epub.load(false, true);
     return RecentBook{path, epub.getTitle(), epub.getAuthor(), epub.getThumbBmpPath()};
   } else if (StringUtils::checkFileExtension(lastBookFileName, ".xtch") ||
              StringUtils::checkFileExtension(lastBookFileName, ".xtc")) {
diff --git a/src/activities/reader/ReaderActivity.cpp b/src/activities/reader/ReaderActivity.cpp
index b336c9a3..f2f9199a 100644
--- a/src/activities/reader/ReaderActivity.cpp
+++ b/src/activities/reader/ReaderActivity.cpp
@@ -2,6 +2,7 @@
 
 #include <HalStorage.h>
 
+#include "CrossPointSettings.h"
 #include "Epub.h"
 #include "EpubReaderActivity.h"
 #include "Txt.h"
@@ -35,7 +36,7 @@ std::unique_ptr<Epub> ReaderActivity::loadEpub(const std::string& path) {
   }
 
   auto epub = std::unique_ptr<Epub>(new Epub(path, "/.crosspoint"));
-  if (epub->load()) {
+  if (epub->load(true, SETTINGS.embeddedStyle == 0)) {
     return epub;
   }