[mathml] Implement script level attribute and font-size extension

MathML Core support the script level functionality both using a
font-size extension and as an attribute on MathML elements.

The scriptlevel attributes simply maps to the font-size extension [1].

The font-size extension follows the algorithm stated in [2].

In order to keep track of the internal scriptlevel [3] an internal property MathScriptLevel is added to the computed style.

Finally various stylesheet rules related to script level are taken as-is from the specification stylesheet [4].

[1] https://mathml-refresh.github.io/mathml-core/#the-displaystyle-and-scriptlevel-attributes
[2] https://mathml-refresh.github.io/mathml-core/#the-math-script-level-property
[3] https://mathml-refresh.github.io/mathml-core/#dfn-internal-scriptlevel
[4] https://mathml-refresh.github.io/mathml-core/#user-agent-stylesheet

Bug: 1023292
Change-Id: I47ed250a63049b15daff53f9c0942cf676b21acd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2184131Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Frédéric Wang <fwang@igalia.com>
Commit-Queue: Rob Buis <rbuis@igalia.com>
Cr-Commit-Position: refs/heads/master@{#770955}

third_party/blink/renderer/core/css/css_value_keywords.json5

@@ -1190,6 +1190,10 @@
     "p3",
     "rec2020",
 
+    // math-script-level
+    "add",
+    "scriptlevel",
+
     // overscroll-behavior
     // auto,
     // contain

third_party/blink/renderer/core/css/mathml.css

@@ -65,15 +65,45 @@ mspace {
     overflow: hidden !important;
 }
 
+/* Token elements */
+mi {
+  text-transform: math-auto;
+}
+
+/* Fractions */
 mfrac {
   padding-inline-start: 1px;
   padding-inline-end: 1px;
 }
 
+mfrac > * {
+  font-size: scriptlevel(auto);
+  math-style: inline;
+}
+
 mfrac > :nth-child(2) {
   math-superscript-shift-style: inline;
 }
 
+/* Other rules for scriptlevel, displaystyle and math-superscript-shift-style */
+msub > :not(:first-child),
+msup > :not(:first-child),
+msubsup > :not(:first-child),
+mmultiscripts > :not(:first-child),
+munder > :not(:first-child),
+mover > :not(:first-child),
+munderover > :not(:first-child) {
+  font-size: scriptlevel(add(1));
+  math-style: inline;
+}
+
+munder[accentunder="true" i] > :nth-child(2),
+mover[accent="true" i] > :nth-child(2),
+munderover[accentunder="true" i] > :nth-child(2),
+munderover[accent="true" i] > :nth-child(3) {
+  font-size: inherit;
+}
+
 munder > :nth-child(2),
 munderover > :nth-child(2),
 mover[accent="true" i] > :first-child,
@@ -81,8 +111,3 @@ munderover[accent="true" i] > :first-child {
   math-superscript-shift-style: inline;
 }
 
-/* Token elements */
-mi {
-  text-transform: math-auto;
-}
-

third_party/blink/renderer/core/css/properties/css_parsing_utils.cc

@@ -1340,6 +1340,39 @@ CSSValue* ConsumeCounter(CSSParserTokenRange& range,
   return list;
 }
 
+CSSValue* ConsumeScriptLevel(CSSParserTokenRange& range,
+                             const CSSParserContext& context) {
+  CSSValueID function_id = range.Peek().FunctionId();
+  DCHECK(function_id == CSSValueID::kScriptlevel);
+  CSSParserTokenRange args =
+      css_property_parser_helpers::ConsumeFunction(range);
+  if (args.AtEnd())
+    return nullptr;
+  CSSValue* parsed_value =
+      css_property_parser_helpers::ConsumeIdent<CSSValueID::kAuto>(args);
+  if (!parsed_value)
+    parsed_value = css_property_parser_helpers::ConsumeInteger(args, context);
+  if (!parsed_value) {
+    function_id = args.Peek().FunctionId();
+    if (function_id == CSSValueID::kAdd) {
+      auto* add_value = MakeGarbageCollected<CSSFunctionValue>(function_id);
+      CSSParserTokenRange add_args =
+          css_property_parser_helpers::ConsumeFunction(args);
+      if ((parsed_value = css_property_parser_helpers::ConsumeInteger(
+               add_args, context))) {
+        add_value->Append(*parsed_value);
+        parsed_value = add_value;
+      }
+    }
+  }
+  if (!parsed_value || !args.AtEnd())
+    return nullptr;
+  auto* script_level_value =
+      MakeGarbageCollected<CSSFunctionValue>(CSSValueID::kScriptlevel);
+  script_level_value->Append(*parsed_value);
+  return script_level_value;
+}
+
 CSSValue* ConsumeFontSize(CSSParserTokenRange& range,
                           const CSSParserContext& context,
                           css_property_parser_helpers::UnitlessQuirk unitless) {
@@ -1348,6 +1381,9 @@ CSSValue* ConsumeFontSize(CSSParserTokenRange& range,
   if (range.Peek().Id() >= CSSValueID::kXxSmall &&
       range.Peek().Id() <= CSSValueID::kWebkitXxxLarge)
     return css_property_parser_helpers::ConsumeIdent(range);
+  if (RuntimeEnabledFeatures::CSSMathStyleEnabled() &&
+      range.Peek().FunctionId() == CSSValueID::kScriptlevel)
+    return ConsumeScriptLevel(range, context);
   return css_property_parser_helpers::ConsumeLengthOrPercent(
       range, context, kValueRangeNonNegative, unitless);
 }

third_party/blink/renderer/core/css/resolver/style_builder_converter.cc

@@ -59,6 +59,7 @@
 #include "third_party/blink/renderer/core/style/reference_clip_path_operation.h"
 #include "third_party/blink/renderer/core/style/shape_clip_path_operation.h"
 #include "third_party/blink/renderer/core/style/style_svg_resource.h"
+#include "third_party/blink/renderer/platform/fonts/opentype/open_type_math_support.h"
 #include "third_party/blink/renderer/platform/heap/heap.h"
 #include "third_party/blink/renderer/platform/instrumentation/use_counter.h"
 
@@ -293,6 +294,58 @@ StyleBuilderConverter::ConvertFontVariationSettings(StyleResolverState& state,
   return settings;
 }
 
+float MathScriptScaleFactor(StyleResolverState& state) {
+  int a = state.ParentStyle()->MathScriptLevel();
+  int b = state.Style()->MathScriptLevel();
+  if (b == a)
+    return 1.0;
+  bool invertScaleFactor = false;
+  if (b < a) {
+    std::swap(a, b);
+    invertScaleFactor = true;
+  }
+
+  // Determine the scale factors from the inherited font.
+  float defaultScaleDown = 0.71;
+  int exponent = b - a;
+  float scaleFactor = 1.0;
+  HarfBuzzFace* parent_harfbuzz_face = state.ParentStyle()
+                                           ->GetFont()
+                                           .PrimaryFont()
+                                           ->PlatformData()
+                                           .GetHarfBuzzFace();
+  if (OpenTypeMathSupport::HasMathData(parent_harfbuzz_face)) {
+    float scriptPercentScaleDown =
+        OpenTypeMathSupport::MathConstant(
+            parent_harfbuzz_face,
+            OpenTypeMathSupport::MathConstants::kScriptPercentScaleDown)
+            .value_or(0);
+    // Note: zero can mean both zero for the math constant and the fallback.
+    if (!scriptPercentScaleDown)
+      scriptPercentScaleDown = defaultScaleDown;
+    float scriptScriptPercentScaleDown =
+        OpenTypeMathSupport::MathConstant(
+            parent_harfbuzz_face,
+            OpenTypeMathSupport::MathConstants::kScriptScriptPercentScaleDown)
+            .value_or(0);
+    // Note: zero can mean both zero for the math constant and the fallback.
+    if (!scriptScriptPercentScaleDown)
+      scriptScriptPercentScaleDown = defaultScaleDown * defaultScaleDown;
+    if (a <= 0 && b >= 2) {
+      scaleFactor *= scriptScriptPercentScaleDown;
+      exponent -= 2;
+    } else if (a == 1) {
+      scaleFactor *= scriptScriptPercentScaleDown / scriptPercentScaleDown;
+      exponent--;
+    } else if (b == 1) {
+      scaleFactor *= scriptPercentScaleDown;
+      exponent--;
+    }
+  }
+  scaleFactor *= pow(defaultScaleDown, exponent);
+  return invertScaleFactor ? 1 / scaleFactor : scaleFactor;
+}
+
 static float ComputeFontSize(const CSSToLengthConversionData& conversion_data,
                              const CSSPrimitiveValue& primitive_value,
                              const FontDescription::Size& parent_size) {
@@ -356,14 +409,48 @@ FontDescription::Size StyleBuilderConverterBase::ConvertFontSize(
       is_absolute);
 }
 
+static FontDescription::Size ConvertScriptLevelFontSize(
+    StyleResolverState& state,
+    const FontDescription::Size& parent_size,
+    const CSSFunctionValue& function_value) {
+  SECURITY_DCHECK(function_value.length() == 1);
+  const auto& css_value = function_value.Item(0);
+  if (const auto* list = DynamicTo<CSSValueList>(css_value)) {
+    DCHECK_EQ(list->length(), 1U);
+    const auto& relative_value = To<CSSPrimitiveValue>(list->Item(0));
+    state.Style()->SetMathScriptLevel(state.ParentStyle()->MathScriptLevel() +
+                                      relative_value.GetIntValue());
+  } else if (auto* identifier_value =
+                 DynamicTo<CSSIdentifierValue>(css_value)) {
+    DCHECK(identifier_value->GetValueID() == CSSValueID::kAuto);
+    unsigned level = 0;
+    if (state.ParentStyle()->MathStyle() == EMathStyle::kInline)
+      level += 1;
+    state.Style()->SetMathScriptLevel(state.ParentStyle()->MathScriptLevel() +
+                                      level);
+  } else if (DynamicTo<CSSPrimitiveValue>(css_value)) {
+    state.Style()->SetMathScriptLevel(
+        To<CSSPrimitiveValue>(css_value).GetIntValue());
+  }
+  auto scale_factor = MathScriptScaleFactor(state);
+  state.Style()->SetHasGlyphRelativeUnits();
+  return FontDescription::Size(0, (scale_factor * parent_size.value),
+                               parent_size.is_absolute);
+}
+
 FontDescription::Size StyleBuilderConverter::ConvertFontSize(
     StyleResolverState& state,
     const CSSValue& value) {
+  // FIXME: Find out when parentStyle could be 0?
+  auto parent_size = state.ParentStyle()
+                         ? state.ParentFontDescription().GetSize()
+                         : FontDescription::Size(0, 0.0f, false);
+
+  if (const auto* function_value = DynamicTo<CSSFunctionValue>(value))
+    return ConvertScriptLevelFontSize(state, parent_size, *function_value);
+
   return StyleBuilderConverterBase::ConvertFontSize(
-      value, state.FontSizeConversionData(),
-      // FIXME: Find out when parentStyle could be 0?
-      state.ParentStyle() ? state.ParentFontDescription().GetSize()
-                          : FontDescription::Size(0, 0.0f, false));
+      value, state.FontSizeConversionData(), parent_size);
 }
 
 float StyleBuilderConverter::ConvertFontSizeAdjust(StyleResolverState& state,

third_party/blink/renderer/core/mathml/mathml_attribute_names.json5

@@ -18,6 +18,7 @@
     "mathcolor",
     "mathsize",
     "mathvariant",
+    "scriptlevel",
     "width",
   ],
 }

third_party/blink/renderer/core/mathml/mathml_element.cc

@@ -34,7 +34,6 @@ static inline bool IsDisallowedMathSizeAttribute(const AtomicString& value) {
 }
 
 bool MathMLElement::IsPresentationAttribute(const QualifiedName& name) const {
-  // TODO(crbug.com/1023292): add support for displaystyle and scriptlevel.
   if (name == html_names::kDirAttr || name == mathml_names::kMathsizeAttr ||
       name == mathml_names::kMathcolorAttr ||
       name == mathml_names::kMathbackgroundAttr ||
@@ -49,8 +48,6 @@ void MathMLElement::CollectStyleForPresentationAttribute(
     const QualifiedName& name,
     const AtomicString& value,
     MutableCSSPropertyValueSet* style) {
-  // TODO(crbug.com/1023292, crbug.com/1023296): add support for display,
-  // displaystyle and scriptlevel.
   if (name == html_names::kDirAttr) {
     if (IsValidDirAttribute(value)) {
       AddPropertyToPresentationAttributeStyle(style, CSSPropertyID::kDirection,

third_party/blink/renderer/core/style/computed_style_extra_fields.json5

@@ -1128,5 +1128,13 @@
       field_group: "*",
       getter: "GetMathFractionBarThickness",
     },
+    {
+      name: "MathScriptLevel",
+      inherited: true,
+      field_template: "primitive",
+      type_name: "short",
+      default_value: "0",
+      field_group: "*",
+    },
   ],
 }

third_party/blink/web_tests/TestExpectations

@@ -1274,15 +1274,6 @@ crbug.com/626703 external/wpt/css/css-text/text-transform/math/text-transform-ma
 crbug.com/626703 external/wpt/css/css-text/text-transform/math/text-transform-math-script-001.tentative.html [ Failure ]
 crbug.com/626703 external/wpt/css/css-text/text-transform/math/text-transform-math-sans-serif-bold-italic-001.tentative.html [ Failure ]
 crbug.com/626703 external/wpt/css/css-text/text-transform/math/text-transform-math-sans-serif-001.tentative.html [ Failure ]
-crbug.com/626703 [ Linux ] external/wpt/css/css-fonts/math-script-level-and-math-style/math-script-level-auto-and-math-style-002.tentative.html [ Failure ]
-crbug.com/626703 [ Mac ] external/wpt/css/css-fonts/math-script-level-and-math-style/math-script-level-auto-and-math-style-002.tentative.html [ Failure ]
-crbug.com/626703 [ Win ] external/wpt/css/css-fonts/math-script-level-and-math-style/math-script-level-auto-and-math-style-002.tentative.html [ Failure ]
-crbug.com/626703 [ Linux ] external/wpt/css/css-fonts/math-script-level-and-math-style/math-script-level-auto-and-math-style-003.tentative.html [ Failure ]
-crbug.com/626703 [ Mac ] external/wpt/css/css-fonts/math-script-level-and-math-style/math-script-level-auto-and-math-style-003.tentative.html [ Failure ]
-crbug.com/626703 [ Win ] external/wpt/css/css-fonts/math-script-level-and-math-style/math-script-level-auto-and-math-style-003.tentative.html [ Failure ]
-crbug.com/626703 [ Linux ] external/wpt/css/css-fonts/math-script-level-and-math-style/math-script-level-auto-and-math-style-004.tentative.html [ Failure ]
-crbug.com/626703 [ Mac ] external/wpt/css/css-fonts/math-script-level-and-math-style/math-script-level-auto-and-math-style-004.tentative.html [ Failure ]
-crbug.com/626703 [ Win ] external/wpt/css/css-fonts/math-script-level-and-math-style/math-script-level-auto-and-math-style-004.tentative.html [ Failure ]
 crbug.com/1043295 [ Fuchsia ] virtual/font-access/http/tests/font-access/font-access-window.html [ Skip ]
 crbug.com/1043295 [ Fuchsia ] virtual/font-access/http/tests/font-access/font-access-workers.html [ Skip ]
 

third_party/blink/web_tests/external/wpt/css/css-fonts/math-script-level-and-math-style/math-script-level-002.tentative-expected.txt

-This is a testharness.js-based test.
-FAIL auto assert_equals: expected 355 but got 500
-FAIL auto ; starting from level 7 assert_approx_equals: expected 181.90240316781995 +/- 0.1 but got 2000
-FAIL add(<integer>) assert_equals: expected 100 but got 71
-FAIL add(<integer>) ; starting from level 3 assert_equals: expected 100 but got 71
-FAIL <integer> assert_equals: expected 100 but got 71
-FAIL <integer> ; starting from level 50 assert_equals: expected 100 but got 71
-Harness: the test ran to completion.
-

third_party/blink/web_tests/external/wpt/css/css-fonts/math-script-level-and-math-style/math-script-level-004.tentative-expected.txt

-This is a testharness.js-based test.
-FAIL scriptPercentScaleDown=80, scriptScriptPercentScaleDown=40 assert_approx_equals: Wrong font-size (id=scale80-40-scaledown ; level=0 ; i=0) expected 1073.733 +/- 1 but got 3000
-FAIL scriptPercentScaleDown=0, scriptScriptPercentScaleDown=40 assert_approx_equals: Wrong font-size (id=scale0-40-scaledown ; level=0 ; i=0) expected 1073.733 +/- 1 but got 3000
-FAIL scriptPercentScaleDown=80, scriptScriptPercentScaleDown=0 assert_approx_equals: Wrong font-size (id=scale80-0-scaledown ; level=0 ; i=0) expected 1073.733 +/- 1 but got 3000
-FAIL No MATH table assert_approx_equals: Wrong font-size (id=default-scaledown ; level=0 ; i=0) expected 1073.733 +/- 1 but got 3000
-Harness: the test ran to completion.
-

third_party/blink/web_tests/external/wpt/mathml/relations/css-styling/scriptlevel-001-expected.txt

 This is a testharness.js-based test.
 PASS automatic scriptlevel on mfrac (displaystyle=true)
-FAIL automatic scriptlevel on mfrac (displaystyle=false) assert_approx_equals: numerator expected 71 +/- 0.1 but got 100
+PASS automatic scriptlevel on mfrac (displaystyle=false)
 FAIL automatic scriptlevel on mroot assert_approx_equals: index expected 50.41 +/- 0.1 but got 100
-FAIL automatic scriptlevel on msub assert_approx_equals: child 1 expected 71 +/- 0.1 but got 100
-FAIL automatic scriptlevel on msup assert_approx_equals: child 1 expected 71 +/- 0.1 but got 100
-FAIL automatic scriptlevel on msubsup assert_approx_equals: child 1 expected 71 +/- 0.1 but got 100
-FAIL automatic scriptlevel on munder assert_approx_equals: child 1 expected 71 +/- 0.1 but got 100
-FAIL automatic scriptlevel on mover assert_approx_equals: child 1 expected 71 +/- 0.1 but got 100
-FAIL automatic scriptlevel on munderover assert_approx_equals: child 1 expected 71 +/- 0.1 but got 100
-FAIL automatic scriptlevel on mmultiscripts assert_approx_equals: child 1 expected 71 +/- 0.1 but got 100
+PASS automatic scriptlevel on msub
+PASS automatic scriptlevel on msup
+PASS automatic scriptlevel on msubsup
+PASS automatic scriptlevel on munder
+PASS automatic scriptlevel on mover
+PASS automatic scriptlevel on munderover
+PASS automatic scriptlevel on mmultiscripts
 PASS automatic scriptlevel on munder (accentunder=true)
 PASS automatic scriptlevel on mover (accent=true)
-FAIL automatic scriptlevel on munderover (accentunder=true) assert_approx_equals: over expected 71 +/- 0.1 but got 100
-FAIL automatic scriptlevel on munderover (accent=true) assert_approx_equals: under expected 71 +/- 0.1 but got 100
-FAIL checking dynamic/case-insensitive accent/accentunder assert_approx_equals: under expected 71 +/- 0.1 but got 100
+PASS automatic scriptlevel on munderover (accentunder=true)
+PASS automatic scriptlevel on munderover (accent=true)
+PASS checking dynamic/case-insensitive accent/accentunder
 Harness: the test ran to completion.