feat: check the formal viewport meta tag syntax

- `HTM-047` is now reported for invalid `viewport` values
- new `HTM-056` is reported when a `viewport` value has no `width` or
  `height`
- new `HTM-057` is reported when a viewport width or height value is
  invalid (i.e. not a positive number or a well-known kewyword)

Also:

- the `isASCIIWhitespace` static utility method is extraced to a public
  `InfraUtil` class in a new `org.w3c.epubcheck.util.infra` package
- the viewport parser is tested as a standalone class in its own feature
  file. However, for easier lookup, this file is however located in the
  `epub3/F-viewport-meta-tag` directory, as if it was a functional test.
- several additional functional tests are added to the `epub3/08-layout`
  directory, for viewport metadata checks.

Fixes #1214, fixes #1347

src/main/java/com/adobe/epubcheck/messages/DefaultSeverities.java

@@ -137,6 +137,8 @@ private void initialize()
     severities.put(MessageId.HTM_053, Severity.INFO);
     severities.put(MessageId.HTM_054, Severity.ERROR);
     severities.put(MessageId.HTM_055, Severity.WARNING);
+    severities.put(MessageId.HTM_056, Severity.ERROR);
+    severities.put(MessageId.HTM_057, Severity.ERROR);
 
     // Media
     severities.put(MessageId.MED_001, Severity.ERROR);

src/main/java/com/adobe/epubcheck/messages/MessageId.java

@@ -131,6 +131,8 @@ public enum MessageId implements Comparable<MessageId>
   HTM_053("HTM_053"),
   HTM_054("HTM_054"),
   HTM_055("HTM_055"),
+  HTM_056("HTM_056"),
+  HTM_057("HTM_057"),
 
   // Messages associated with media (images, audio and video)
   MED_001("MED-001"),

src/main/java/com/adobe/epubcheck/ops/OPSHandler30.java

@@ -3,6 +3,7 @@
 import java.util.Collections;
 import java.util.EnumSet;
 import java.util.HashSet;
+import java.util.LinkedList;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
@@ -11,6 +12,9 @@
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
+import org.w3c.epubcheck.util.microsyntax.ViewportMeta;
+import org.w3c.epubcheck.util.microsyntax.ViewportMeta.ParseError;
+
 import com.adobe.epubcheck.api.EPUBLocation;
 import com.adobe.epubcheck.api.EPUBProfile;
 import com.adobe.epubcheck.messages.MessageId;
@@ -701,19 +705,55 @@ protected void processMeta()
     if (EpubConstants.HtmlNamespaceUri.equals(e.getNamespace()))
     {
       String name = e.getAttribute("name");
-      if ("viewport".equals(name))
+      if ("viewport".equals(Strings.nullToEmpty(name).trim()))
       {
+        // Mark the viewport as seen
+        // (used when checking the existence of viewport metadata)
         hasViewport = true;
-        // For a fixed-layout document:
-        // check that the vieport declares both height and width
+        // For a fixed-layout documents:
         if (context.opfItem.isPresent() && context.opfItem.get().isFixedLayout())
         {
           String contentAttribute = e.getAttribute("content");
-          if (contentAttribute == null || !contentAttribute.contains("width=")
-              || !contentAttribute.contains("height="))
+
+          // parse viewport metadata
+          List<ViewportMeta.ParseError> syntaxErrors = new LinkedList<>();
+          ViewportMeta viewport = ViewportMeta.parse(contentAttribute,
+              new ViewportMeta.ErrorHandler()
+              {
+                @Override
+                public void error(ParseError error, int position)
+                {
+                  syntaxErrors.add(error);
+                }
+              });
+          if (!syntaxErrors.isEmpty())
+          {
+            // report any syntax error
+            report.message(MessageId.HTM_047, location(), contentAttribute);
+          }
+          else
           {
-            report.message(MessageId.HTM_047, location());
+            // check that viewport metadata has a valid width value
+            if (!viewport.hasProperty("width"))
+            {
+              report.message(MessageId.HTM_056, location(), "width");
+            }
+            else if (!ViewportMeta.isValidWidth(viewport.getValue("width")))
+            {
+              report.message(MessageId.HTM_057, location(), "width");
+            }
+
+            // check that viewport metadata has a valid height value
+            if (!viewport.hasProperty("height"))
+            {
+              report.message(MessageId.HTM_056, location(), "height");
+            }
+            else if (!ViewportMeta.isValidHeight(viewport.getValue("height")))
+            {
+              report.message(MessageId.HTM_057, location(), "height");
+            }
           }
+
         }
       }
     }

src/main/java/com/adobe/epubcheck/util/SourceSet.java

@@ -1,10 +1,14 @@
 package com.adobe.epubcheck.util;
 
+import static org.w3c.epubcheck.util.infra.InfraUtil.isASCIIWhitespace;
+
 import java.nio.CharBuffer;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.regex.Pattern;
 
+import org.w3c.epubcheck.util.infra.InfraUtil;
+
 import com.google.common.base.Function;
 import com.google.common.base.Optional;
 import com.google.common.base.Preconditions;
@@ -162,7 +166,7 @@ public SourceSet parse(CharSequence srcset)
         {
         case SPLIT:
           assert (url.length() == 0);
-          if (isASCIIWhitespace(c))
+          if (InfraUtil.isASCIIWhitespace(c))
           {
             // skip whitespace
           }
@@ -271,15 +275,6 @@ else if (c == '(')
       return builder.build();
     }
 
-    /**
-     * if a character is https://infra.spec.whatwg.org/#ascii-whitespace U+0009 TAB,
-     * U+000A LF, U+000C FF, U+000D CR, or U+0020 SPACE.
-     */
-    private static boolean isASCIIWhitespace(char c)
-    {
-      return c == ' ' || c == '\t' || c == '\f' || c == '\n' || c == '\r';
-    }
-
     /**
      * Remove trailing commas and return the count of commas removed
      */

src/main/java/org/w3c/epubcheck/util/infra/InfraUtil.java

@@ -0,0 +1,19 @@
+package org.w3c.epubcheck.util.infra;
+
+public final class InfraUtil
+{
+
+  private InfraUtil()
+  {
+    // static utility class
+  }
+
+  /**
+   * if a character is https://infra.spec.whatwg.org/#ascii-whitespace U+0009
+   * TAB, U+000A LF, U+000C FF, U+000D CR, or U+0020 SPACE.
+   */
+  public static boolean isASCIIWhitespace(char c)
+  {
+    return c == ' ' || c == '\t' || c == '\f' || c == '\n' || c == '\r';
+  }
+}