Issue #2862 - Initial attempt at a css lowering

layout/style/nsCSSParser.cpp

@@ -7,6 +7,7 @@
 
 #include "mozilla/ArrayUtils.h"
 #include "mozilla/DebugOnly.h"
+#include "mozilla/Maybe.h"
 #include "mozilla/Move.h"
 #include "mozilla/MathAlgorithms.h"
 #include "mozilla/TypedEnumBits.h"
@@ -71,6 +72,7 @@ static bool sMozGradientsEnabled;
 static bool sControlCharVisibility;
 static bool sLegacyNegationPseudoClassEnabled;
 static bool sCascadeLayersEnabled;
+static bool sNestingEnabled;
 
 const uint32_t
 nsCSSProps::kParserVariantTable[eCSSProperty_COUNT_no_shorthands] = {
@@ -217,6 +219,843 @@ OKLabToSRGBColor(float aL, float aA, float aB, float aAlpha)
     alpha);
 }
 
+class CSSNestingLowerer final
+{
+  using SelectorList = nsTArray<nsString>;
+
+public:
+  explicit CSSNestingLowerer(const nsAString& aInput)
+    : mInput(aInput)
+    , mPos(0)
+    , mSawNesting(false)
+  {
+  }
+
+  bool Lower(nsAString& aOutput)
+  {
+    nsAutoString lowered;
+    if (!ProcessStylesheet(lowered, false)) {
+      return false;
+    }
+
+    SkipWhitespaceAndComments();
+    if (mPos != mInput.Length() || !mSawNesting) {
+      return false;
+    }
+
+    aOutput.Assign(lowered);
+    return true;
+  }
+
+private:
+  static constexpr auto kCSSWhitespace = " \t\r\n\f";
+
+  static bool
+  IsCSSWhitespace(char16_t aChar)
+  {
+    return aChar == ' ' || aChar == '\t' || aChar == '\r' ||
+           aChar == '\n' || aChar == '\f';
+  }
+
+  bool
+  AtEnd() const
+  {
+    return mPos >= mInput.Length();
+  }
+
+  char16_t
+  Peek() const
+  {
+    MOZ_ASSERT(!AtEnd(), "cannot peek past end");
+    return mInput.CharAt(mPos);
+  }
+
+  bool
+  StartsWithComment() const
+  {
+    return mPos + 1 < mInput.Length() &&
+           mInput.CharAt(mPos) == '/' &&
+           mInput.CharAt(mPos + 1) == '*';
+  }
+
+  bool
+  SkipComment()
+  {
+    MOZ_ASSERT(StartsWithComment(), "expected comment");
+
+    mPos += 2;
+    while (mPos + 1 < mInput.Length()) {
+      if (mInput.CharAt(mPos) == '*' && mInput.CharAt(mPos + 1) == '/') {
+        mPos += 2;
+        return true;
+      }
+      ++mPos;
+    }
+
+    return false;
+  }
+
+  void
+  SkipWhitespaceAndComments()
+  {
+    while (!AtEnd()) {
+      if (IsCSSWhitespace(Peek())) {
+        ++mPos;
+        continue;
+      }
+      if (StartsWithComment()) {
+        if (!SkipComment()) {
+          mPos = mInput.Length();
+          return;
+        }
+        continue;
+      }
+      break;
+    }
+  }
+
+  bool
+  SkipString(char16_t aQuote)
+  {
+    MOZ_ASSERT(!AtEnd() && Peek() == aQuote, "expected string start");
+
+    ++mPos;
+    while (!AtEnd()) {
+      char16_t c = Peek();
+      ++mPos;
+      if (c == aQuote) {
+        return true;
+      }
+      if (c == '\\' && !AtEnd()) {
+        ++mPos;
+        continue;
+      }
+      if (c == '\n' || c == '\r' || c == '\f') {
+        return false;
+      }
+    }
+
+    return false;
+  }
+
+  static void
+  TrimWhitespace(nsAString& aText)
+  {
+    uint32_t start = 0;
+    uint32_t end = aText.Length();
+
+    while (start < end && IsCSSWhitespace(aText.CharAt(start))) {
+      ++start;
+    }
+    while (end > start && IsCSSWhitespace(aText.CharAt(end - 1))) {
+      --end;
+    }
+
+    if (start == 0 && end == aText.Length()) {
+      return;
+    }
+
+    aText.Assign(Substring(aText, start, end - start));
+  }
+
+  bool
+  SplitSelectorList(const nsAString& aSelectorText, SelectorList& aSelectors)
+  {
+    uint32_t itemStart = 0;
+    int32_t parenDepth = 0;
+    int32_t bracketDepth = 0;
+    bool inComment = false;
+    char16_t stringQuote = 0;
+
+    for (uint32_t i = 0; i < aSelectorText.Length(); ++i) {
+      char16_t c = aSelectorText.CharAt(i);
+
+      if (inComment) {
+        if (c == '*' && i + 1 < aSelectorText.Length() &&
+            aSelectorText.CharAt(i + 1) == '/') {
+          inComment = false;
+          ++i;
+        }
+        continue;
+      }
+
+      if (stringQuote) {
+        if (c == '\\') {
+          ++i;
+          continue;
+        }
+        if (c == stringQuote) {
+          stringQuote = 0;
+        }
+        continue;
+      }
+
+      if (c == '/' && i + 1 < aSelectorText.Length() &&
+          aSelectorText.CharAt(i + 1) == '*') {
+        inComment = true;
+        ++i;
+        continue;
+      }
+
+      if (c == '"' || c == '\'') {
+        stringQuote = c;
+        continue;
+      }
+
+      if (c == '(') {
+        ++parenDepth;
+        continue;
+      }
+      if (c == ')' && parenDepth > 0) {
+        --parenDepth;
+        continue;
+      }
+      if (c == '[') {
+        ++bracketDepth;
+        continue;
+      }
+      if (c == ']' && bracketDepth > 0) {
+        --bracketDepth;
+        continue;
+      }
+
+      if (c == ',' && parenDepth == 0 && bracketDepth == 0) {
+        nsAutoString selector;
+        selector.Assign(Substring(aSelectorText, itemStart, i - itemStart));
+        TrimWhitespace(selector);
+        if (!selector.IsEmpty()) {
+          aSelectors.AppendElement(selector);
+        }
+        itemStart = i + 1;
+      }
+    }
+
+    nsAutoString selector;
+    selector.Assign(Substring(aSelectorText, itemStart));
+    TrimWhitespace(selector);
+    if (!selector.IsEmpty()) {
+      aSelectors.AppendElement(selector);
+    }
+
+    return !aSelectors.IsEmpty();
+  }
+
+  bool
+  SelectorHasAmpersand(const nsAString& aSelector) const
+  {
+    bool inComment = false;
+    char16_t stringQuote = 0;
+
+    for (uint32_t i = 0; i < aSelector.Length(); ++i) {
+      char16_t c = aSelector.CharAt(i);
+
+      if (inComment) {
+        if (c == '*' && i + 1 < aSelector.Length() &&
+            aSelector.CharAt(i + 1) == '/') {
+          inComment = false;
+          ++i;
+        }
+        continue;
+      }
+
+      if (stringQuote) {
+        if (c == '\\') {
+          ++i;
+          continue;
+        }
+        if (c == stringQuote) {
+          stringQuote = 0;
+        }
+        continue;
+      }
+
+      if (c == '/' && i + 1 < aSelector.Length() &&
+          aSelector.CharAt(i + 1) == '*') {
+        inComment = true;
+        ++i;
+        continue;
+      }
+
+      if (c == '"' || c == '\'') {
+        stringQuote = c;
+        continue;
+      }
+
+      if (c == '&') {
+        return true;
+      }
+    }
+
+    return false;
+  }
+
+  void
+  ReplaceAmpersands(const nsAString& aSelector,
+                    const nsAString& aParent,
+                    nsAString& aOutput) const
+  {
+    bool inComment = false;
+    char16_t stringQuote = 0;
+
+    for (uint32_t i = 0; i < aSelector.Length(); ++i) {
+      char16_t c = aSelector.CharAt(i);
+
+      if (inComment) {
+        aOutput.Append(c);
+        if (c == '*' && i + 1 < aSelector.Length() &&
+            aSelector.CharAt(i + 1) == '/') {
+          aOutput.Append('/');
+          inComment = false;
+          ++i;
+        }
+        continue;
+      }
+
+      if (stringQuote) {
+        aOutput.Append(c);
+        if (c == '\\' && i + 1 < aSelector.Length()) {
+          aOutput.Append(aSelector.CharAt(i + 1));
+          ++i;
+          continue;
+        }
+        if (c == stringQuote) {
+          stringQuote = 0;
+        }
+        continue;
+      }
+
+      if (c == '/' && i + 1 < aSelector.Length() &&
+          aSelector.CharAt(i + 1) == '*') {
+        aOutput.AppendLiteral("/*");
+        inComment = true;
+        ++i;
+        continue;
+      }
+
+      if (c == '"' || c == '\'') {
+        aOutput.Append(c);
+        stringQuote = c;
+        continue;
+      }
+
+      if (c == '&') {
+        aOutput.Append(aParent);
+        continue;
+      }
+
+      aOutput.Append(c);
+    }
+  }
+
+  bool
+  ExpandNestedSelectors(const SelectorList& aParents,
+                        const nsAString& aNestedSelectorText,
+                        SelectorList& aSelectors)
+  {
+    SelectorList nestedSelectors;
+    if (!SplitSelectorList(aNestedSelectorText, nestedSelectors)) {
+      return false;
+    }
+
+    for (const nsString& nestedSelector : nestedSelectors) {
+      bool hasAmpersand = SelectorHasAmpersand(nestedSelector);
+      for (const nsString& parentSelector : aParents) {
+        nsAutoString combined;
+        if (hasAmpersand) {
+          ReplaceAmpersands(nestedSelector, parentSelector, combined);
+        } else {
+          combined.Assign(parentSelector);
+          if (!combined.IsEmpty()) {
+            combined.Append(' ');
+          }
+          combined.Append(nestedSelector);
+        }
+        TrimWhitespace(combined);
+        if (!combined.IsEmpty()) {
+          aSelectors.AppendElement(combined);
+        }
+      }
+    }
+
+    return !aSelectors.IsEmpty();
+  }
+
+  static void
+  AppendSelectors(const SelectorList& aSelectors, nsAString& aOutput)
+  {
+    for (uint32_t i = 0; i < aSelectors.Length(); ++i) {
+      if (i) {
+        aOutput.AppendLiteral(", ");
+      }
+      aOutput.Append(aSelectors[i]);
+    }
+  }
+
+  static bool
+  StartsNestedSelector(char16_t aChar)
+  {
+    switch (aChar) {
+      case '.':
+      case '#':
+      case '[':
+      case ':':
+      case '&':
+      case '>':
+      case '+':
+      case '~':
+      case '*':
+        return true;
+      default:
+        return false;
+    }
+  }
+
+  static bool
+  IsAtRuleNameChar(char16_t aChar)
+  {
+    return (aChar >= 'a' && aChar <= 'z') ||
+           (aChar >= 'A' && aChar <= 'Z') ||
+           (aChar >= '0' && aChar <= '9') ||
+           aChar == '-';
+  }
+
+  static void
+  LowercaseASCII(nsACString& aText)
+  {
+    for (uint32_t i = 0; i < aText.Length(); ++i) {
+      char c = aText.CharAt(i);
+      if (c >= 'A' && c <= 'Z') {
+        aText.BeginWriting()[i] = c - 'A' + 'a';
+      }
+    }
+  }
+
+  static bool
+  ShouldProcessGroupRule(const nsACString& aName)
+  {
+    return aName.EqualsLiteral("media") ||
+           aName.EqualsLiteral("supports") ||
+           aName.EqualsLiteral("document") ||
+           aName.EqualsLiteral("layer");
+  }
+
+  void
+  FlushDeclarations(const SelectorList& aSelectors,
+                    nsAString& aDeclarations,
+                    nsAString& aOutput)
+  {
+    nsAutoString declarations;
+    declarations.Assign(aDeclarations);
+    TrimWhitespace(declarations);
+    aDeclarations.Truncate();
+
+    if (declarations.IsEmpty()) {
+      return;
+    }
+
+    AppendSelectors(aSelectors, aOutput);
+    aOutput.AppendLiteral(" { ");
+    aOutput.Append(declarations);
+    aOutput.AppendLiteral(" }\n");
+  }
+
+  bool
+  ReadRawBlockBody(nsAString& aBody)
+  {
+    uint32_t start = mPos;
+    int32_t depth = 0;
+
+    while (!AtEnd()) {
+      char16_t c = Peek();
+      if (c == '"' || c == '\'') {
+        if (!SkipString(c)) {
+          return false;
+        }
+        continue;
+      }
+      if (StartsWithComment()) {
+        if (!SkipComment()) {
+          return false;
+        }
+        continue;
+      }
+      if (c == '{') {
+        ++depth;
+        ++mPos;
+        continue;
+      }
+      if (c == '}') {
+        if (depth == 0) {
+          aBody.Assign(Substring(mInput, start, mPos - start));
+          ++mPos;
+          return true;
+        }
+        --depth;
+        ++mPos;
+        continue;
+      }
+      ++mPos;
+    }
+
+    return false;
+  }
+
+  bool
+  ReadQualifiedRulePrelude(nsAString& aPrelude)
+  {
+    uint32_t start = mPos;
+    int32_t parenDepth = 0;
+    int32_t bracketDepth = 0;
+
+    while (!AtEnd()) {
+      char16_t c = Peek();
+      if (c == '"' || c == '\'') {
+        if (!SkipString(c)) {
+          return false;
+        }
+        continue;
+      }
+      if (StartsWithComment()) {
+        if (!SkipComment()) {
+          return false;
+        }
+        continue;
+      }
+      if (c == '(') {
+        ++parenDepth;
+        ++mPos;
+        continue;
+      }
+      if (c == ')' && parenDepth > 0) {
+        --parenDepth;
+        ++mPos;
+        continue;
+      }
+      if (c == '[') {
+        ++bracketDepth;
+        ++mPos;
+        continue;
+      }
+      if (c == ']' && bracketDepth > 0) {
+        --bracketDepth;
+        ++mPos;
+        continue;
+      }
+      if (c == '{' && parenDepth == 0 && bracketDepth == 0) {
+        aPrelude.Assign(Substring(mInput, start, mPos - start));
+        TrimWhitespace(aPrelude);
+        ++mPos;
+        return !aPrelude.IsEmpty();
+      }
+      if ((c == ';' || c == '}') && parenDepth == 0 && bracketDepth == 0) {
+        return false;
+      }
+      ++mPos;
+    }
+
+    return false;
+  }
+
+  bool
+  ReadAtRulePrelude(nsAString& aPrelude, nsACString& aName, bool& aHasBlock)
+  {
+    MOZ_ASSERT(!AtEnd() && Peek() == '@', "expected at-rule");
+
+    uint32_t start = mPos;
+    ++mPos;
+    aName.Truncate();
+    while (!AtEnd() && IsAtRuleNameChar(Peek())) {
+      char16_t c = Peek();
+      aName.Append(char(c <= 0x7f ? c : '?'));
+      ++mPos;
+    }
+    LowercaseASCII(aName);
+
+    int32_t parenDepth = 0;
+    int32_t bracketDepth = 0;
+    while (!AtEnd()) {
+      char16_t c = Peek();
+      if (c == '"' || c == '\'') {
+        if (!SkipString(c)) {
+          return false;
+        }
+        continue;
+      }
+      if (StartsWithComment()) {
+        if (!SkipComment()) {
+          return false;
+        }
+        continue;
+      }
+      if (c == '(') {
+        ++parenDepth;
+        ++mPos;
+        continue;
+      }
+      if (c == ')' && parenDepth > 0) {
+        --parenDepth;
+        ++mPos;
+        continue;
+      }
+      if (c == '[') {
+        ++bracketDepth;
+        ++mPos;
+        continue;
+      }
+      if (c == ']' && bracketDepth > 0) {
+        --bracketDepth;
+        ++mPos;
+        continue;
+      }
+      if (parenDepth == 0 && bracketDepth == 0) {
+        if (c == ';') {
+          aPrelude.Assign(Substring(mInput, start, mPos - start));
+          TrimWhitespace(aPrelude);
+          ++mPos;
+          aHasBlock = false;
+          return true;
+        }
+        if (c == '{') {
+          aPrelude.Assign(Substring(mInput, start, mPos - start));
+          TrimWhitespace(aPrelude);
+          ++mPos;
+          aHasBlock = true;
+          return true;
+        }
+      }
+      ++mPos;
+    }
+
+    return false;
+  }
+
+  bool
+  ConsumeDeclaration(nsAString& aDeclaration)
+  {
+    uint32_t start = mPos;
+    int32_t parenDepth = 0;
+    int32_t bracketDepth = 0;
+    int32_t braceDepth = 0;
+
+    while (!AtEnd()) {
+      char16_t c = Peek();
+      if (c == '"' || c == '\'') {
+        if (!SkipString(c)) {
+          return false;
+        }
+        continue;
+      }
+      if (StartsWithComment()) {
+        if (!SkipComment()) {
+          return false;
+        }
+        continue;
+      }
+      if (c == '(') {
+        ++parenDepth;
+        ++mPos;
+        continue;
+      }
+      if (c == ')' && parenDepth > 0) {
+        --parenDepth;
+        ++mPos;
+        continue;
+      }
+      if (c == '[') {
+        ++bracketDepth;
+        ++mPos;
+        continue;
+      }
+      if (c == ']' && bracketDepth > 0) {
+        --bracketDepth;
+        ++mPos;
+        continue;
+      }
+      if (c == '{') {
+        ++braceDepth;
+        ++mPos;
+        continue;
+      }
+      if (c == '}') {
+        if (braceDepth == 0 && parenDepth == 0 && bracketDepth == 0) {
+          break;
+        }
+        if (braceDepth > 0) {
+          --braceDepth;
+        }
+        ++mPos;
+        continue;
+      }
+      if (c == ';' && parenDepth == 0 && bracketDepth == 0 &&
+          braceDepth == 0) {
+        ++mPos;
+        break;
+      }
+      ++mPos;
+    }
+
+    aDeclaration.Assign(Substring(mInput, start, mPos - start));
+    TrimWhitespace(aDeclaration);
+    if (aDeclaration.IsEmpty()) {
+      return false;
+    }
+    if (aDeclaration.Last() != ';') {
+      aDeclaration.Append(';');
+    }
+    return true;
+  }
+
+  bool
+  ParseAtRule(nsAString& aOutput, const SelectorList* aParents)
+  {
+    nsAutoString prelude;
+    nsAutoCString name;
+    bool hasBlock = false;
+    if (!ReadAtRulePrelude(prelude, name, hasBlock)) {
+      return false;
+    }
+
+    if (!hasBlock) {
+      aOutput.Append(prelude);
+      aOutput.AppendLiteral(";\n");
+      return true;
+    }
+
+    if (!ShouldProcessGroupRule(name)) {
+      nsAutoString body;
+      if (!ReadRawBlockBody(body)) {
+        return false;
+      }
+      aOutput.Append(prelude);
+      aOutput.AppendLiteral(" {");
+      aOutput.Append(body);
+      aOutput.AppendLiteral("}\n");
+      return true;
+    }
+
+    nsAutoString inner;
+    if (aParents) {
+      mSawNesting = true;
+      if (!ProcessStyleContext(*aParents, inner)) {
+        return false;
+      }
+    } else {
+      if (!ProcessStylesheet(inner, true)) {
+        return false;
+      }
+    }
+
+    aOutput.Append(prelude);
+    aOutput.AppendLiteral(" {\n");
+    aOutput.Append(inner);
+    aOutput.AppendLiteral("}\n");
+    return true;
+  }
+
+  bool
+  ParseQualifiedRule(nsAString& aOutput, const SelectorList* aParents)
+  {
+    nsAutoString prelude;
+    if (!ReadQualifiedRulePrelude(prelude)) {
+      return false;
+    }
+
+    SelectorList selectors;
+    if (aParents) {
+      mSawNesting = true;
+      if (!ExpandNestedSelectors(*aParents, prelude, selectors)) {
+        return false;
+      }
+    } else if (!SplitSelectorList(prelude, selectors)) {
+      return false;
+    }
+
+    return ProcessStyleContext(selectors, aOutput);
+  }
+
+  bool
+  ProcessStyleContext(const SelectorList& aSelectors, nsAString& aOutput)
+  {
+    nsAutoString declarations;
+
+    while (!AtEnd()) {
+      SkipWhitespaceAndComments();
+      if (AtEnd()) {
+        return false;
+      }
+
+      char16_t c = Peek();
+      if (c == '}') {
+        ++mPos;
+        FlushDeclarations(aSelectors, declarations, aOutput);
+        return true;
+      }
+
+      if (c == '@') {
+        FlushDeclarations(aSelectors, declarations, aOutput);
+        if (!ParseAtRule(aOutput, &aSelectors)) {
+          return false;
+        }
+        continue;
+      }
+
+      if (StartsNestedSelector(c)) {
+        FlushDeclarations(aSelectors, declarations, aOutput);
+        if (!ParseQualifiedRule(aOutput, &aSelectors)) {
+          return false;
+        }
+        continue;
+      }
+
+      nsAutoString declaration;
+      if (!ConsumeDeclaration(declaration)) {
+        return false;
+      }
+      if (!declarations.IsEmpty()) {
+        declarations.Append(' ');
+      }
+      declarations.Append(declaration);
+    }
+
+    return false;
+  }
+
+  bool
+  ProcessStylesheet(nsAString& aOutput, bool aStopAtBlockEnd)
+  {
+    while (!AtEnd()) {
+      SkipWhitespaceAndComments();
+      if (AtEnd()) {
+        return !aStopAtBlockEnd;
+      }
+
+      if (Peek() == '}') {
+        if (!aStopAtBlockEnd) {
+          return false;
+        }
+        ++mPos;
+        return true;
+      }
+
+      if (Peek() == '@') {
+        if (!ParseAtRule(aOutput, nullptr)) {
+          return false;
+        }
+      } else {
+        if (!ParseQualifiedRule(aOutput, nullptr)) {
+          return false;
+        }
+      }
+    }
+
+    return !aStopAtBlockEnd;
+  }
+
+  const nsAString& mInput;
+  uint32_t mPos;
+  bool mSawNesting;
+};
+
 static_assert(css::eAuthorSheetFeatures == 0 &&
               css::eUserSheetFeatures == 1 &&
               css::eAgentSheetFeatures == 2,
@@ -1838,7 +2677,16 @@ CSSParserImpl::ParseSheet(const nsAString& aInput,
                "Sheet principal does not match passed principal");
 #endif
 
-  nsCSSScanner scanner(aInput, aLineNumber);
+  nsAutoString loweredInput;
+  const nsAString* input = &aInput;
+  if (sNestingEnabled) {
+    CSSNestingLowerer lowerer(aInput);
+    if (lowerer.Lower(loweredInput)) {
+      input = &loweredInput;
+    }
+  }
+
+  nsCSSScanner scanner(*input, aLineNumber);
   css::ErrorReporter reporter(scanner, mSheet, mChildLoader, aSheetURI);
   InitScanner(scanner, reporter, aSheetURI, aBaseURI, aSheetPrincipal);
 
@@ -19024,6 +19872,8 @@ nsCSSParser::Startup()
                                "layout.css.legacy-negation-pseudo.enabled");
   Preferences::AddBoolVarCache(&sCascadeLayersEnabled,
                                "layout.css.cascade-layers.enabled");
+  Preferences::AddBoolVarCache(&sNestingEnabled,
+                               "layout.css.nesting.enabled");
 }
 
 nsCSSParser::nsCSSParser(mozilla::css::Loader* aLoader,

modules/libpref/init/all.js

@@ -2711,6 +2711,9 @@ pref("layout.css.resizeobserver.enabled", true);
 // Is support for cascade layers enabled?
 pref("layout.css.cascade-layers.enabled", true);
 
+// Is support for basic CSS nesting lowering enabled?
+pref("layout.css.nesting.enabled", false);
+
 // Should rules in imported style sheets be added based on the order
 // of appearance of their respective @import rules in the parent
 // style sheet? Otherwise, they are added before rules preceding
@@ -3216,7 +3219,7 @@ pref("ui.mouse.radius.inputSource.touchOnly", true);
 
 #ifdef XP_WIN
 
-// Be as uniform as possible, use Twemoji everywhere. 
+// Be as uniform as possible, use Twemoji everywhere.
 // Optional: prefix with `Segoe UI Emoji` to use Win8+ Segoe UI font emoji where available.
 pref("font.name-list.emoji", "Twemoji Mozilla");
 
@@ -4731,7 +4734,7 @@ pref("media.ondevicechange.fakeDeviceChangeEvent.enabled", false);
 // those platforms we don't handle touch events anyway so it's conceptually
 // a no-op.
 pref("layout.css.touch_action.enabled", true);
- 
+
 // WHATWG computed intrinsic aspect ratio for an img element
 // https://html.spec.whatwg.org/multipage/rendering.html#attributes-for-embedded-content-and-images
 // Are the width and height attributes on image-like elements mapped to the
@@ -5245,7 +5248,7 @@ pref("plugins.navigator_hide_disabled_flash", false);
 pref("dom.mozBrowserFramesEnabled", false);
 
 // Thick caret when behind CJK characters
-pref("layout.cjkthickcaret", true); 
+pref("layout.cjkthickcaret", true);
 
 // Is support for 'color-adjust' CSS property enabled?
 pref("layout.css.color-adjust.enabled", true);