Add Helvetica and sans-serif as fallback font families

lib/web_ui/lib/src/engine/text/paragraph.dart

@@ -1085,7 +1085,7 @@ void _applyParagraphStyleToElement({
           style._fontStyle == ui.FontStyle.normal ? 'normal' : 'italic';
     }
     if (style._effectiveFontFamily != null) {
-      cssStyle.fontFamily = quoteFontFamily(style._effectiveFontFamily);
+      cssStyle.fontFamily = canonicalizeFontFamily(style._effectiveFontFamily);
     }
   } else {
     if (style._textAlign != previousStyle._textAlign) {
@@ -1111,7 +1111,7 @@ void _applyParagraphStyleToElement({
           : null;
     }
     if (style._fontFamily != previousStyle._fontFamily) {
-      cssStyle.fontFamily = quoteFontFamily(style._fontFamily);
+      cssStyle.fontFamily = canonicalizeFontFamily(style._fontFamily);
     }
   }
 }
@@ -1151,11 +1151,11 @@ void _applyTextStyleToElement({
     // consistently use Ahem font.
     if (isSpan && !ui.debugEmulateFlutterTesterEnvironment) {
       if (style._fontFamily != null) {
-        cssStyle.fontFamily = quoteFontFamily(style._fontFamily);
+        cssStyle.fontFamily = canonicalizeFontFamily(style._fontFamily);
       }
     } else {
       if (style._effectiveFontFamily != null) {
-        cssStyle.fontFamily = quoteFontFamily(style._effectiveFontFamily);
+        cssStyle.fontFamily = canonicalizeFontFamily(style._effectiveFontFamily);
       }
     }
     if (style._letterSpacing != null) {
@@ -1189,7 +1189,7 @@ void _applyTextStyleToElement({
           : null;
     }
     if (style._fontFamily != previousStyle._fontFamily) {
-      cssStyle.fontFamily = quoteFontFamily(style._fontFamily);
+      cssStyle.fontFamily = canonicalizeFontFamily(style._fontFamily);
     }
     if (style._letterSpacing != previousStyle._letterSpacing) {
       cssStyle.letterSpacing = '${style._letterSpacing}px';

lib/web_ui/lib/src/engine/text/ruler.dart

@@ -86,7 +86,7 @@ class ParagraphGeometricStyle {
       result.write(DomRenderer.defaultFontSize);
     }
     result.write(' ');
-    result.write(quoteFontFamily(effectiveFontFamily));
+    result.write(canonicalizeFontFamily(effectiveFontFamily));
 
     return result.toString();
   }
@@ -227,7 +227,7 @@ class TextDimensions {
   void applyStyle(ParagraphGeometricStyle style) {
     _element.style
       ..fontSize = style.fontSize != null ? '${style.fontSize.floor()}px' : null
-      ..fontFamily = quoteFontFamily(style.effectiveFontFamily)
+      ..fontFamily = canonicalizeFontFamily(style.effectiveFontFamily)
       ..fontWeight =
           style.fontWeight != null ? fontWeightToCss(style.fontWeight) : null
       ..fontStyle = style.fontStyle != null

lib/web_ui/lib/src/engine/util.dart

@@ -258,10 +258,20 @@ const Set<String> _genericFontFamilies = <String>{
   'fangsong',
 };
 
-/// Wraps a font family in quotes unless it is a generic font family.
-String quoteFontFamily(String fontFamily) {
+/// A default fallback font family in case an unloaded font has been requested.
+///
+/// For iOS, default to Helvetica, where it should be available, otherwise
+/// default to Arial.
+final String _fallbackFontFamily =
+    operatingSystem == OperatingSystem.iOs ? 'Helvetica' : 'Arial';
+
+/// Create a font-family string appropriate for CSS.
+///
+/// If the given [fontFamily] is a generic font-family, then just return it.
+/// Otherwise, wrap the family name in quotes and add a fallback font family.
+String canonicalizeFontFamily(String fontFamily) {
   if (_genericFontFamilies.contains(fontFamily)) {
     return fontFamily;
   }
-  return '"$fontFamily"';
+  return '"$fontFamily", $_fallbackFontFamily, sans-serif';
 }

lib/web_ui/test/text_test.dart

@@ -241,9 +241,43 @@ void main() async {
     expect(paragraph.plainText, isNull);
     final List<SpanElement> spans =
         paragraph.paragraphElement.querySelectorAll('span');
-    expect(spans[0].style.fontFamily, 'Ahem');
+    expect(spans[0].style.fontFamily, 'Ahem, Arial, sans-serif');
     // The nested span here should not set it's family to default sans-serif.
-    expect(spans[1].style.fontFamily, 'Ahem');
+    expect(spans[1].style.fontFamily, 'Ahem, Arial, sans-serif');
+  });
+
+  test('adds Arial and sans-serif as fallback fonts', () {
+    // Set this to false so it doesn't default to 'Ahem' font.
+    debugEmulateFlutterTesterEnvironment = false;
+
+    final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(
+      fontFamily: 'SomeFont',
+      fontSize: 12.0,
+    ));
+
+    builder.addText('Hello');
+
+    final EngineParagraph paragraph = builder.build();
+    expect(paragraph.paragraphElement.style.fontFamily, 'SomeFont, Arial, sans-serif');
+
+    debugEmulateFlutterTesterEnvironment = true;
+  });
+
+  test('does not add fallback fonts to generic families', () {
+    // Set this to false so it doesn't default to 'Ahem' font.
+    debugEmulateFlutterTesterEnvironment = false;
+
+    final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(
+      fontFamily: 'serif',
+      fontSize: 12.0,
+    ));
+
+    builder.addText('Hello');
+
+    final EngineParagraph paragraph = builder.build();
+    expect(paragraph.paragraphElement.style.fontFamily, 'serif');
+
+    debugEmulateFlutterTesterEnvironment = true;
   });
 
   test('can set font families that need to be quoted', () {
@@ -258,10 +292,11 @@ void main() async {
     builder.addText('Hello');
 
     final EngineParagraph paragraph = builder.build();
-    expect(paragraph.paragraphElement.style.fontFamily, '"MyFont 2000"');
+    expect(paragraph.paragraphElement.style.fontFamily, '"MyFont 2000", Arial, sans-serif');
 
     debugEmulateFlutterTesterEnvironment = true;
   });
+
   group('TextRange', () {
     test('empty ranges are correct', () {
       const TextRange range = TextRange(start: -1, end: -1);