fix: improve checking of missing SVG link labels, now USAGE

This commit changes the severity of error `ACC-011` to USAGE (was WARNING).
Labelling links is not strictly a spec requirement, and checking for
missing labels should be done with specialized accessibility inspection.

The check is still available, with an improved logic: the link is
reported as having a missing accessible label only if:
- it does not have an `xlink:title` attribute
- AND it does not have an `aria-label` attribute
- AND it does not have a `title` element child
- AND it does not have a `text` element child

Note that his logic is incomplete, as it does not check that the content
of `title` or `text` children is not empty. But reporting links that
have none of the above can be somewhat useful as a simplified first-pass
accessibility check.

Fix #1353

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

@@ -48,7 +48,7 @@ private void initialize()
     severities.put(MessageId.ACC_008, Severity.SUPPRESSED);
     severities.put(MessageId.ACC_009, Severity.USAGE);
     severities.put(MessageId.ACC_010, Severity.SUPPRESSED);
-    severities.put(MessageId.ACC_011, Severity.WARNING);
+    severities.put(MessageId.ACC_011, Severity.USAGE);
     severities.put(MessageId.ACC_012, Severity.SUPPRESSED);
     severities.put(MessageId.ACC_013, Severity.SUPPRESSED);
     severities.put(MessageId.ACC_014, Severity.SUPPRESSED);

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

@@ -93,7 +93,7 @@ public class OPSHandler30 extends OPSHandler
   protected boolean inRegionBasedNav = false;
   protected boolean isOutermostSVGAlreadyProcessed = false;
   protected boolean hasAltorAnnotation = false;
-  protected boolean hasTitle = false;
+  protected boolean hasLabel = false;
   protected boolean hasViewport = false;
 
   static protected final String[] scriptEventsStrings = { "onafterprint", "onbeforeprint",
@@ -399,7 +399,11 @@ else if (name.equals("source"))
     }
     else if ("http://www.w3.org/2000/svg".equals(e.getNamespace()) && name.equals("title"))
     {
-      hasTitle = true;
+      hasLabel = true;
+    }
+    else if ("http://www.w3.org/2000/svg".equals(e.getNamespace()) && name.equals("text"))
+    {
+      hasLabel = true;
     }
 
     processInlineScripts();
@@ -491,8 +495,9 @@ protected void processAnchor(XMLElement e)
     }
     if (inSvg || context.mimeType.equals("image/svg+xml"))
     {
-      hasTitle = Strings
-          .emptyToNull(e.getAttributeNS(EpubConstants.XLinkNamespaceUri, "title")) != null;
+      String title = e.getAttributeNS(EpubConstants.XLinkNamespaceUri, "title");
+      String ariaLabel = e.getAttribute("aria-label");
+      hasLabel = !Strings.isNullOrEmpty(title) || !Strings.isNullOrEmpty(ariaLabel);
     }
   }
 
@@ -866,7 +871,7 @@ else if (name.equals("a"))
         report.message(MessageId.ACC_004, location().context("a"));
         anchorNeedsText = false;
       }
-      if ((inSvg || context.mimeType.equals("image/svg+xml")) && !hasTitle)
+      if ((inSvg || context.mimeType.equals("image/svg+xml")) && !hasLabel)
       {
         report.message(MessageId.ACC_011, location().context(e.getName()));
       }

src/main/resources/com/adobe/epubcheck/messages/MessageBundle.properties

@@ -14,7 +14,7 @@ ACC_007=Content Documents do not use "epub:type" attributes for semantic inflect
 ACC_008=Navigation Document has no "landmarks nav" element.
 ACC_009=MathML should either have an "alttext" attribute or "annotation-xml" child element.
 ACC_010=Headings should not be used within blockquote and figure elements.
-ACC_011=SVG hyperlinks should have a human-readable title (using the "title" child element or the "xlink:title" attribute).
+ACC_011=SVG hyperlink has no accessible name
 ACC_012=Table elements should include a caption element.
 ACC_013=Content file contains at least one inline style declaration.
 ACC_013_SUG=Inline styles are not compatible with accessibility settings and display personalization. Use CSS Styles instead.

src/test/java/org/w3c/epubcheck/test/AssertionSteps.java

@@ -40,6 +40,12 @@ public void assertNoErrorsOrWarning()
     assertThat("Unexpected warning", report.getAll(Severity.WARNING), is(emptyIterable()));
   }
 
+  @Then("no( other) usage(s) are/is reported")
+  public void assertNoUsage()
+  {
+    assertThat("Unexpected usage", report.getAll(Severity.USAGE), is(emptyIterable()));
+  }
+
   /*
    * Common step definition for "is reported" and "is reported {int} times" see
    * https://github.com/cucumber/cucumber-expressions/issues/166

src/test/resources/epub3/06-content-document/content-document-svg.feature

@@ -84,9 +84,11 @@ Feature: EPUB 3 — Content Documents — SVG
     When checking document 'link-valid.svg'
     Then no errors or warnings are reported
 
-  Scenario: Report SVG link without a title
-    When checking document 'link-no-title-error.svg'
-    Then warning ACC-011 is reported
+  Scenario: Report SVG link without a label as usage
+    Given the reporting level is set to usage
+    When checking document 'link-label-valid.svg'
+    Then usage ACC-011 is reported
+    And no other usages are reported
     And no other errors or warnings are reported
 
   Scenario: Verify that `image` elements can have an `xlink:href` URL pointing to a fragment 

src/test/resources/epub3/06-content-document/content-document-xhtml.feature

@@ -961,12 +961,6 @@ Feature: EPUB 3 — Content Documents — XHTML
     And the message contains 'element "foo" not allowed here'
     And no other errors or warnings are reported
 
-  #TODO review if this warning is relevant
-  Scenario: Report an SVG link without a recommended title
-    When checking document 'svg-links-no-title-warning.xhtml'
-    Then warning ACC-011 is reported
-    And no other errors or warnings are reported
-
   Scenario: Verify unprefixed HTML elements allowed inside prefixed `foreignObject`
     When checking document 'svg-foreignObject-valid.xhtml'
     Then no errors or warnings are reported