Introduce letter-spacing-override descriptor for @font-face (1/2)

Design doc: https://bit.ly/39qATQ4

This patch adds a new @font-face descriptor to override spacing between
characters when the font is used.

For simplicity, this patch only implements the parsing of the
descriptor, but doesn't affect any rendering. A follow up patch
(crrev.com/c/2318510) will apply it in text shaping.

Note: Though we don't have a stable spec at the moment, we land
it behind a flag to make testing and discussion easier.

Bug: 1098355
Change-Id: I7921aad0d03e0b7da0c2fca77c99324ab7449e2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2335823Reviewed-by: Dominik Röttsches <drott@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794696}

third_party/blink/public/mojom/use_counter/css_property_id.mojom

@@ -717,6 +717,7 @@ enum CSSSampleId {
     kScrollbarGutter = 671,
     kAscentOverride = 672,
     kDescentOverride = 673,
+    kLetterSpacingOverride = 674,
     // 1. Add new features above this line (don't change the assigned numbers of
     //    the existing items).
     // 2. Run the src/tools/metrics/histograms/update_use_counter_css.py script

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

@@ -5133,6 +5133,12 @@
       is_property: false,
       runtime_flag: "CSSFontMetricsOverride",
     },
+    {
+      name: "letter-spacing-override",
+      is_descriptor: true,
+      is_property: false,
+      runtime_flag: "CSSFontMetricsOverride",
+    },
 
     // Shorthands
     {

third_party/blink/renderer/core/css/font_face.cc

@@ -195,6 +195,8 @@ FontFace* FontFace::Create(Document* document,
                                       AtRuleDescriptorID::AscentOverride) &&
       font_face->SetPropertyFromStyle(properties,
                                       AtRuleDescriptorID::DescentOverride) &&
+      font_face->SetPropertyFromStyle(
+          properties, AtRuleDescriptorID::LetterSpacingOverride) &&
       font_face->GetFontSelectionCapabilities().IsValid() &&
       !font_face->family().IsEmpty()) {
     font_face->InitCSSFontFace(document->GetExecutionContext(), *src);
@@ -363,6 +365,9 @@ bool FontFace::SetPropertyValue(const CSSValue* value,
     case AtRuleDescriptorID::DescentOverride:
       descent_override_ = value;
       break;
+    case AtRuleDescriptorID::LetterSpacingOverride:
+      letter_spacing_override_ = value;
+      break;
     default:
       NOTREACHED();
       return false;
@@ -806,6 +811,7 @@ void FontFace::Trace(Visitor* visitor) const {
   visitor->Trace(display_);
   visitor->Trace(ascent_override_);
   visitor->Trace(descent_override_);
+  visitor->Trace(letter_spacing_override_);
   visitor->Trace(error_);
   visitor->Trace(loaded_property_);
   visitor->Trace(css_font_face_);
@@ -843,6 +849,10 @@ FontMetricsOverride FontFace::GetFontMetricsOverride() const {
     result.descent_override =
         To<CSSPrimitiveValue>(*descent_override_).GetFloatValue() / 100;
   }
+  if (letter_spacing_override_) {
+    result.letter_spacing_override =
+        To<CSSPrimitiveValue>(*letter_spacing_override_).GetFloatValue();
+  }
   return result;
 }
 

third_party/blink/renderer/core/css/font_face.h

@@ -135,7 +135,7 @@ class CORE_EXPORT FontFace : public ScriptWrappable,
   bool HasPendingActivity() const final;
 
   bool HasFontMetricsOverride() const {
-    return ascent_override_ || descent_override_;
+    return ascent_override_ || descent_override_ || letter_spacing_override_;
   }
   FontMetricsOverride GetFontMetricsOverride() const;
 
@@ -179,6 +179,7 @@ class CORE_EXPORT FontFace : public ScriptWrappable,
   Member<const CSSValue> display_;
   Member<const CSSValue> ascent_override_;
   Member<const CSSValue> descent_override_;
+  Member<const CSSValue> letter_spacing_override_;
   LoadStatusType status_;
   Member<DOMException> error_;
 

third_party/blink/renderer/core/css/parser/at_rule_descriptor_parser.cc

@@ -208,6 +208,13 @@ CSSValue* ConsumeFontMetricOverride(CSSParserTokenRange& range,
                                            kValueRangeNonNegative);
 }
 
+CSSValue* ConsumeLetterSpacingOverride(CSSParserTokenRange& range,
+                                       const CSSParserContext& context) {
+  if (!RuntimeEnabledFeatures::CSSFontMetricsOverrideEnabled())
+    return nullptr;
+  return css_parsing_utils::ConsumeNumber(range, context, kValueRangeAll);
+}
+
 }  // namespace
 
 CSSValue* AtRuleDescriptorParser::ParseFontFaceDescriptor(
@@ -261,6 +268,9 @@ CSSValue* AtRuleDescriptorParser::ParseFontFaceDescriptor(
     case AtRuleDescriptorID::DescentOverride:
       parsed_value = ConsumeFontMetricOverride(range, context);
       break;
+    case AtRuleDescriptorID::LetterSpacingOverride:
+      parsed_value = ConsumeLetterSpacingOverride(range, context);
+      break;
     default:
       break;
   }

third_party/blink/renderer/core/css/parser/at_rule_names.json5

@@ -54,6 +54,9 @@
     {
       name: "initial-value"
     },
+    {
+      name: "letter-spacing-override"
+    },
     {
       name: "max-height",
     },

third_party/blink/renderer/platform/fonts/font_metrics_override.h

@@ -12,6 +12,7 @@ namespace blink {
 struct FontMetricsOverride {
   base::Optional<float> ascent_override;
   base::Optional<float> descent_override;
+  base::Optional<float> letter_spacing_override;
 };
 
 }  // namespace blink

third_party/blink/web_tests/webexposed/css-properties-as-js-properties-expected.txt

@@ -213,6 +213,7 @@ justifySelf
 left
 length
 letterSpacing
+letterSpacingOverride
 lightingColor
 lineBreak
 lineHeight

tools/metrics/histograms/enums.xml

@@ -45259,6 +45259,7 @@ Called by update_use_counter_css.py.-->
   <int value="671" label="scrollbar-gutter"/>
   <int value="672" label="ascent-override"/>
   <int value="673" label="descent-override"/>
+  <int value="674" label="letter-spacing-override"/>
 </enum>
 
 <enum name="MappedEditingCommands">