--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1,8 +1,10 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
 # This file is automatically @generated by Cargo.
 # It is not intended for manual editing.
 [[package]]
 name = "Inflector"
 version = "0.11.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "lazy_static 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",

--- a/layout/generic/nsColumnSetFrame.cpp
+++ b/layout/generic/nsColumnSetFrame.cpp
@@ -279,23 +279,19 @@ static nscoord GetColumnGap(nsColumnSetF
   const auto& columnGap = aFrame->StylePosition()->mColumnGap;
   if (columnGap.GetUnit() == eStyleUnit_Normal) {
     return aFrame->StyleFont()->mFont.size;
   }
   return nsLayoutUtils::ResolveGapToLength(columnGap, aPercentageBasis);
 }
 
 /* static */
-nscoord nsColumnSetFrame::ClampUsedColumnWidth(
-    const nsStyleCoord& aColumnWidth) {
-  MOZ_ASSERT(aColumnWidth.GetUnit() == eStyleUnit_Coord,
-             "This should only be called when column-width is a <length>!");
-
+nscoord nsColumnSetFrame::ClampUsedColumnWidth(const Length& aColumnWidth) {
   // Per spec, used values will be clamped to a minimum of 1px.
-  return std::max(CSSPixel::ToAppUnits(1), aColumnWidth.GetCoordValue());
+  return std::max(CSSPixel::ToAppUnits(1), aColumnWidth.ToAppUnits());
 }
 
 nsColumnSetFrame::ReflowConfig nsColumnSetFrame::ChooseColumnStrategy(
     const ReflowInput& aReflowInput, bool aForceAuto = false) {
   WritingMode wm = aReflowInput.GetWritingMode();
 
   const nsStyleColumn* colStyle = StyleColumn();
   nscoord availContentISize = GetAvailableContentISize(aReflowInput);
@@ -336,18 +332,18 @@ nsColumnSetFrame::ReflowConfig nsColumnS
     if (cnt == MAX_NESTED_COLUMN_BALANCING) {
       numColumns = 1;
     }
   }
 
   nscoord colISize;
   // In vertical writing-mode, "column-width" (inline size) will actually be
   // physical height, but its CSS name is still column-width.
-  if (colStyle->mColumnWidth.GetUnit() == eStyleUnit_Coord) {
-    colISize = ClampUsedColumnWidth(colStyle->mColumnWidth);
+  if (colStyle->mColumnWidth.IsLength()) {
+    colISize = ClampUsedColumnWidth(colStyle->mColumnWidth.AsLength());
     NS_ASSERTION(colISize >= 0, "negative column width");
     // Reduce column count if necessary to make columns fit in the
     // available width. Compute max number of columns that fit in
     // availContentISize, satisfying colGap*(maxColumns - 1) +
     // colISize*maxColumns <= availContentISize
     if (availContentISize != NS_INTRINSICSIZE && colGap + colISize > 0 &&
         numColumns > 0) {
       // This expression uses truncated rounding, which is what we
@@ -486,18 +482,18 @@ nscoord nsColumnSetFrame::GetMinISize(gf
   if (mFrames.FirstChild() && !StyleDisplay()->IsContainSize()) {
     // We want to ignore this in the case that we're size contained
     // because our children should not contribute to our
     // intrinsic size.
     iSize = mFrames.FirstChild()->GetMinISize(aRenderingContext);
   }
   const nsStyleColumn* colStyle = StyleColumn();
   nscoord colISize;
-  if (colStyle->mColumnWidth.GetUnit() == eStyleUnit_Coord) {
-    colISize = ClampUsedColumnWidth(colStyle->mColumnWidth);
+  if (colStyle->mColumnWidth.IsLength()) {
+    colISize = ClampUsedColumnWidth(colStyle->mColumnWidth.AsLength());
     // As available width reduces to zero, we reduce our number of columns
     // to one, and don't enforce the column width, so just return the min
     // of the child's min-width with any specified column width.
     iSize = std::min(iSize, colISize);
   } else {
     NS_ASSERTION(colStyle->mColumnCount > 0,
                  "column-count and column-width can't both be auto");
     // As available width reduces to zero, we still have mColumnCount columns,
@@ -522,18 +518,18 @@ nscoord nsColumnSetFrame::GetPrefISize(g
   // of any required column gaps
   // XXX what about forced column breaks here?
   nscoord result = 0;
   DISPLAY_PREF_INLINE_SIZE(this, result);
   const nsStyleColumn* colStyle = StyleColumn();
   nscoord colGap = GetColumnGap(this, NS_UNCONSTRAINEDSIZE);
 
   nscoord colISize;
-  if (colStyle->mColumnWidth.GetUnit() == eStyleUnit_Coord) {
-    colISize = ClampUsedColumnWidth(colStyle->mColumnWidth);
+  if (colStyle->mColumnWidth.IsLength()) {
+    colISize = ClampUsedColumnWidth(colStyle->mColumnWidth.AsLength());
   } else if (mFrames.FirstChild() && !StyleDisplay()->IsContainSize()) {
     // We want to ignore this in the case that we're size contained
     // because our children should not contribute to our
     // intrinsic size.
     colISize = mFrames.FirstChild()->GetPrefISize(aRenderingContext);
   } else {
     colISize = 0;
   }

--- a/layout/generic/nsColumnSetFrame.h
+++ b/layout/generic/nsColumnSetFrame.h
@@ -213,12 +213,12 @@ class nsColumnSetFrame final : public ns
                       const ReflowInput& aReflowInput, nsReflowStatus& aStatus,
                       const ReflowConfig& aConfig, bool aLastColumnUnbounded,
                       ColumnBalanceData& aColData);
 
   void ForEachColumnRule(
       const std::function<void(const nsRect& lineRect)>& aSetLineRect,
       const nsPoint& aPt);
 
-  static nscoord ClampUsedColumnWidth(const nsStyleCoord& aColumnWidth);
+  static nscoord ClampUsedColumnWidth(const mozilla::Length& aColumnWidth);
 };
 
 #endif  // nsColumnSetFrame_h___

--- a/layout/style/ServoCSSPropList.mako.py
+++ b/layout/style/ServoCSSPropList.mako.py
@@ -71,17 +71,16 @@ LONGHANDS_NOT_SERIALIZED_WITH_SERVO = [
     "border-image-width",
     "border-spacing",
     "box-shadow",
     "caret-color",
     "color",
     "column-count",
     "column-gap",
     "column-rule-width",
-    "column-width",
     "display",
     "fill",
     "filter",
     "flex-basis",
     "flex-grow",
     "flex-shrink",
     "grid-auto-columns",
     "grid-auto-flow",

--- a/layout/style/nsComputedDOMStyle.cpp
+++ b/layout/style/nsComputedDOMStyle.cpp
@@ -1060,25 +1060,16 @@ already_AddRefed<CSSValue> nsComputedDOM
     val->SetIdent(eCSSKeyword_auto);
   } else {
     val->SetNumber(column->mColumnCount);
   }
 
   return val.forget();
 }
 
-already_AddRefed<CSSValue> nsComputedDOMStyle::DoGetColumnWidth() {
-  RefPtr<nsROCSSPrimitiveValue> val = new nsROCSSPrimitiveValue;
-
-  // XXX fix the auto case. When we actually have a column frame, I think
-  // we should return the computed column width.
-  SetValueToCoord(val, StyleColumn()->mColumnWidth, true);
-  return val.forget();
-}
-
 already_AddRefed<CSSValue> nsComputedDOMStyle::DoGetColumnRuleWidth() {
   RefPtr<nsROCSSPrimitiveValue> val = new nsROCSSPrimitiveValue;
   val->SetAppUnits(StyleColumn()->GetComputedColumnRuleWidth());
   return val.forget();
 }
 
 static Position MaybeResolvePositionForTransform(const LengthPercentage& aX,
                                                  const LengthPercentage& aY,

--- a/layout/style/nsComputedDOMStyle.h
+++ b/layout/style/nsComputedDOMStyle.h
@@ -326,17 +326,16 @@ class nsComputedDOMStyle final : public 
   already_AddRefed<CSSValue> DoGetScrollbarColor();
 
   /* User interface properties */
   already_AddRefed<CSSValue> DoGetCaretColor();
   already_AddRefed<CSSValue> DoGetForceBrokenImageIcon();
 
   /* Column properties */
   already_AddRefed<CSSValue> DoGetColumnCount();
-  already_AddRefed<CSSValue> DoGetColumnWidth();
   already_AddRefed<CSSValue> DoGetColumnRuleWidth();
 
   /* CSS Transitions */
   already_AddRefed<CSSValue> DoGetTransitionProperty();
   already_AddRefed<CSSValue> DoGetTransitionDuration();
   already_AddRefed<CSSValue> DoGetTransitionDelay();
 
   /* CSS Animations */

--- a/layout/style/nsStyleCoord.h
+++ b/layout/style/nsStyleCoord.h
@@ -39,16 +39,20 @@ enum LogicalCorner {
   eLogicalCornerBEndIStart = 3
 };
 
 using LengthPercentage = StyleLengthPercentage;
 using LengthPercentageOrAuto = StyleLengthPercentageOrAuto;
 using NonNegativeLengthPercentage = StyleNonNegativeLengthPercentage;
 using NonNegativeLengthPercentageOrAuto =
     StyleNonNegativeLengthPercentageOrAuto;
+using Length = StyleLength;
+using LengthOrAuto = StyleLengthOrAuto;
+using NonNegativeLength = StyleNonNegativeLength;
+using NonNegativeLengthOrAuto = StyleNonNegativeLengthOrAuto;
 using BorderRadius = StyleBorderRadius;
 
 bool StyleCSSPixelLength::IsZero() const { return _0 == 0.0f; }
 
 nscoord StyleCSSPixelLength::ToAppUnits() const {
   // We want to resolve the length part of the calc() expression rounding 0.5
   // away from zero, instead of the default behavior of
   // NSToCoordRound{,WithClamp} which do floor(x + 0.5).
@@ -189,16 +193,26 @@ nscoord LengthPercentage::Resolve(nscoor
     return AsLengthPercentage().ToPercentage();                             \
   }
 
 IMPL_LENGTHPERCENTAGE_FORWARDS(LengthPercentageOrAuto)
 IMPL_LENGTHPERCENTAGE_FORWARDS(StyleSize)
 IMPL_LENGTHPERCENTAGE_FORWARDS(StyleMaxSize)
 
 template <>
+inline bool LengthOrAuto::IsLength() const {
+  return IsLengthPercentage();
+}
+
+template <>
+inline const Length& LengthOrAuto::AsLength() const {
+  return AsLengthPercentage();
+}
+
+template <>
 inline bool StyleFlexBasis::IsAuto() const {
   return IsSize() && AsSize().IsAuto();
 }
 
 template <>
 inline bool StyleSize::BehavesLikeInitialValueOnBlockAxis() const {
   return IsAuto() || IsExtremumLength();
 }

--- a/layout/style/nsStyleStruct.cpp
+++ b/layout/style/nsStyleStruct.cpp
@@ -583,17 +583,17 @@ nsChangeHint nsStyleXUL::CalcDifference(
 
 // --------------------
 // nsStyleColumn
 //
 /* static */ const uint32_t nsStyleColumn::kMaxColumnCount;
 /* static */ const uint32_t nsStyleColumn::kColumnCountAuto;
 
 nsStyleColumn::nsStyleColumn(const Document& aDocument)
-    : mColumnWidth(eStyleUnit_Auto),
+    : mColumnWidth(LengthOrAuto::Auto()),
       mColumnRuleColor(StyleColor::CurrentColor()),
       mColumnRuleStyle(StyleBorderStyle::None),
       mColumnRuleWidth(kMediumBorderWidth),
       mTwipsPerPixel(TwipsPerPixel(aDocument)) {
   MOZ_COUNT_CTOR(nsStyleColumn);
 }
 
 nsStyleColumn::~nsStyleColumn() { MOZ_COUNT_DTOR(nsStyleColumn); }
@@ -607,18 +607,17 @@ nsStyleColumn::nsStyleColumn(const nsSty
       mColumnSpan(aSource.mColumnSpan),
       mColumnRuleWidth(aSource.mColumnRuleWidth),
       mTwipsPerPixel(aSource.mTwipsPerPixel) {
   MOZ_COUNT_CTOR(nsStyleColumn);
 }
 
 nsChangeHint nsStyleColumn::CalcDifference(
     const nsStyleColumn& aNewData) const {
-  if ((mColumnWidth.GetUnit() == eStyleUnit_Auto) !=
-          (aNewData.mColumnWidth.GetUnit() == eStyleUnit_Auto) ||
+  if (mColumnWidth.IsAuto() != aNewData.mColumnWidth.IsAuto() ||
       mColumnCount != aNewData.mColumnCount ||
       mColumnSpan != aNewData.mColumnSpan) {
     // We force column count changes to do a reframe, because it's tricky to
     // handle some edge cases where the column count gets smaller and content
     // overflows.
     // XXX not ideal
     return nsChangeHint_ReconstructFrame;
   }

--- a/layout/style/nsStyleStruct.h
+++ b/layout/style/nsStyleStruct.h
@@ -2603,30 +2603,29 @@ struct MOZ_NEEDS_MEMMOVABLE_MEMBERS nsSt
   // This is the maximum number of columns we can process. It's used in
   // nsColumnSetFrame.
   static const uint32_t kMaxColumnCount = 1000;
 
   // This represents the value of column-count: auto.
   static const uint32_t kColumnCountAuto = 0;
 
   uint32_t mColumnCount = kColumnCountAuto;
-  nsStyleCoord mColumnWidth;  // coord, auto
+  mozilla::NonNegativeLengthOrAuto mColumnWidth;
 
   mozilla::StyleColor mColumnRuleColor;
   mozilla::StyleBorderStyle mColumnRuleStyle;  // StyleborderStyle::*
   mozilla::StyleColumnFill mColumnFill = mozilla::StyleColumnFill::Balance;
   mozilla::StyleColumnSpan mColumnSpan = mozilla::StyleColumnSpan::None;
 
   nscoord GetComputedColumnRuleWidth() const {
     return (IsVisibleBorderStyle(mColumnRuleStyle) ? mColumnRuleWidth : 0);
   }
 
   bool IsColumnContainerStyle() const {
-    return (mColumnCount != kColumnCountAuto ||
-            mColumnWidth.GetUnit() != eStyleUnit_Auto);
+    return mColumnCount != kColumnCountAuto || !mColumnWidth.IsAuto();
   }
 
   bool IsColumnSpanStyle() const {
     return mColumnSpan == mozilla::StyleColumnSpan::All;
   }
 
  protected:
   nscoord mColumnRuleWidth;  // coord

--- a/servo/components/style/properties/gecko.mako.rs
+++ b/servo/components/style/properties/gecko.mako.rs
@@ -1199,17 +1199,16 @@ impl Clone for ${style_struct.gecko_stru
 <%def name="impl_trait(style_struct_name, skip_longhands='')">
 <%
     style_struct = next(x for x in data.style_structs if x.name == style_struct_name)
     longhands = [x for x in style_struct.longhands
                 if not (skip_longhands == "*" or x.name in skip_longhands.split())]
 
     # Types used with predefined_type()-defined properties that we can auto-generate.
     predefined_types = {
-        "length::NonNegativeLengthOrAuto": impl_style_coord,
         "length::NonNegativeLengthPercentageOrNormal": impl_style_coord,
         "Length": impl_absolute_length,
         "MozScriptMinSize": impl_absolute_length,
         "RGBAColor": impl_rgba_color,
         "SVGLength": impl_svg_length,
         "SVGOpacity": impl_svg_opacity,
         "SVGPaint": impl_svg_paint,
         "SVGWidth": impl_svg_length,

--- a/servo/ports/geckolib/cbindgen.toml
+++ b/servo/ports/geckolib/cbindgen.toml
@@ -77,16 +77,18 @@ include = [
   "Resize",
   "Overflow",
   "LengthPercentage",
   "LetterSpacing",
   "NonNegativeLengthPercentage",
   "LengthPercentageOrAuto",
   "LineHeight",
   "NonNegativeLengthPercentageOrAuto",
+  "LengthOrAuto",
+  "NonNegativeLengthOrAuto",
   "Rect",
   "IntersectionObserverRootMargin",
   "Size",
   "MaxSize",
   "FlexBasis",
   "Position",
   "BackgroundSize",
   "BorderImageSlice",
@@ -194,16 +196,20 @@ renaming_overrides_prefixing = true
 
 "GenericLengthPercentageOrAuto" = """
   inline bool ConvertsToLength() const;
   inline nscoord ToLength() const;
   inline bool ConvertsToPercentage() const;
   inline float ToPercentage() const;
   inline bool HasPercent() const;
   inline bool HasLengthAndPercentage() const;
+
+  // Just some convenient aliases for LengthOrAuto, to avoid confusing naming.
+  inline bool IsLength() const;
+  inline const StyleLength& AsLength() const;
 """
 
 "GenericSize" = """
   inline bool ConvertsToLength() const;
   inline nscoord ToLength() const;
   inline bool ConvertsToPercentage() const;
   inline float ToPercentage() const;
   inline bool HasPercent() const;