[mathml] Add fallback MathML stylesheet

Blink currently does not properly render MathML but allows authors to specify
fallback content via an annotation child of the <semantics> element. This is
implemented by CSS rules in a dedicated UA sheet. This CL adds a
mathml-fallback.css UA sheet with the same CSS rules and uses it when
MathMLCore is disabled, in order to preserve existing behavior in that case.
When MathMLCore is enabled, the existing mathml.css UA sheet is still used
for now but its content will be modified in future CLs when the MathML
implementation is refined.

Bug: 6606

Change-Id: I4d68f10185d7e25f64bffeee0da5a184fac39932
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1904186Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Frédéric Wang <fwang@igalia.com>
Commit-Queue: Rob Buis <rbuis@igalia.com>
Cr-Commit-Position: refs/heads/master@{#713907}

third_party/blink/public/blink_resources.grd

@@ -25,6 +25,7 @@
       <include name="IDR_UASTYLE_THEME_FORCED_COLORS_CSS" file="../renderer/core/html/resources/forced_colors.css" type="BINDATA" compress="gzip"/>
       <include name="IDR_UASTYLE_SVG_CSS" file="../renderer/core/css/svg.css" type="BINDATA" compress="gzip"/>
       <include name="IDR_UASTYLE_MATHML_CSS" file="../renderer/core/css/mathml.css" type="BINDATA" compress="gzip"/>
+      <include name="IDR_UASTYLE_MATHML_FALLBACK_CSS" file="../renderer/core/css/mathml-fallback.css" type="BINDATA" compress="gzip"/>
       <include name="IDR_UASTYLE_FULLSCREEN_CSS" file="../renderer/core/css/fullscreen.css" type="BINDATA" compress="gzip"/>
       <include name="IDR_UASTYLE_XHTMLMP_CSS" file="../renderer/core/css/xhtmlmp.css" type="BINDATA" compress="gzip"/>
       <include name="IDR_UASTYLE_VIEWPORT_ANDROID_CSS" file="../renderer/core/css/viewportAndroid.css" type="BINDATA" compress="gzip"/>

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

@@ -215,8 +215,10 @@ bool CSSDefaultStyleSheets::EnsureDefaultStyleSheetsForElement(
   // FIXME: We should assert that the sheet only styles MathML elements.
   if (element.namespaceURI() == mathml_names::kNamespaceURI &&
       !mathml_style_sheet_) {
-    mathml_style_sheet_ =
-        ParseUASheet(UncompressResourceAsASCIIString(IDR_UASTYLE_MATHML_CSS));
+    mathml_style_sheet_ = ParseUASheet(
+        RuntimeEnabledFeatures::MathMLCoreEnabled()
+            ? UncompressResourceAsASCIIString(IDR_UASTYLE_MATHML_CSS)
+            : UncompressResourceAsASCIIString(IDR_UASTYLE_MATHML_FALLBACK_CSS));
     default_style_->AddRulesFromSheet(MathmlStyleSheet(), ScreenEval());
     default_print_style_->AddRulesFromSheet(MathmlStyleSheet(), PrintEval());
     changed_default_style = true;

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

+/*
+ * The default style sheet used to render MathML (MathMLCore disabled).
+ *
+ * Copyright (C) 2014 Google Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE COMPUTER, INC. ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE COMPUTER, INC. OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
+ * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+@namespace "http://www.w3.org/1998/Math/MathML";
+
+/* By default, we only display the MathML formulas without any formatting other than the one specified by the display attribute. */
+math {
+    display: inline;
+}
+
+math[display="block"] {
+    display: block;
+    text-align: center;
+}
+
+/* We hide the PresentationExpression constructions that are children of a <semantics> element.
+   http://www.w3.org/TR/MathML/appendixa.html#parsing_PresentationExpression */
+semantics > mi, semantics > mn, semantics > mo, semantics > mtext, semantics > mspace, semantics > ms, semantics > maligngroup, semantics > malignmark, semantics > mrow, semantics > mfrac, semantics > msqrt, semantics > mroot, semantics > mstyle, semantics > merror, semantics > mpadded, semantics > mphantom, semantics > mfenced, semantics > menclose, semantics > msub, semantics > msup, semantics > msubsup, semantics > munder, semantics > mover, semantics > munderover, semantics > mmultiscripts, semantics > mtable, semantics > mstack, semantics > mlongdiv, semantics > maction {
+    display: none;
+}
+
+/* However, we display all the annotations. */
+annotation, annotation-xml {
+    display: inline-block;
+}

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

 /*
- * The default style sheet used to render MathML.
+ * The default style sheet used to render MathML (MathMLCore enabled).
  *
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *