fix: allow unknown alternate style sheet class names

A class name found on a `link` element representing a style sheet was
reported as an error when it was not one of the known keyword defined
in [EPUB Alternate Style Tags](https://idpf.org/epub/altss-tags/).

This commit fixes that by allowing any name in the class attribute.

This commit also downgrades `CSS-005` to a usage; it is reported when
two mutually-exclusive class names are used on the same link element.
EPUB Alternate Style Tags defines these as mutual exclusive names, but
it is mostly unimplemeneted and this was never strictly forbidden.

Fix #989

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

@@ -71,7 +71,7 @@ private void initialize()
     severities.put(MessageId.CSS_002, Severity.ERROR);
     severities.put(MessageId.CSS_003, Severity.WARNING);
     severities.put(MessageId.CSS_004, Severity.ERROR);
-    severities.put(MessageId.CSS_005, Severity.ERROR);
+    severities.put(MessageId.CSS_005, Severity.USAGE);
     severities.put(MessageId.CSS_006, Severity.USAGE);
     severities.put(MessageId.CSS_007, Severity.INFO);
     severities.put(MessageId.CSS_008, Severity.ERROR);

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

@@ -46,6 +46,7 @@
 import com.adobe.epubcheck.vocab.StagingEdupubVocab;
 import com.adobe.epubcheck.vocab.StructureVocab;
 import com.adobe.epubcheck.vocab.StructureVocab.EPUB_TYPES;
+import com.adobe.epubcheck.vocab.UncheckedVocab;
 import com.adobe.epubcheck.vocab.Vocab;
 import com.adobe.epubcheck.vocab.VocabUtil;
 import com.adobe.epubcheck.xml.model.XMLAttribute;
@@ -69,7 +70,7 @@ public class OPSHandler30 extends OPSHandler
       MagazineNavigationVocab.PREFIX, MagazineNavigationVocab.VOCAB, ForeignVocabs.PRISM_PREFIX,
       ForeignVocabs.PRISM_VOCAB);
   private static Map<String, Vocab> ALTCSS_VOCABS = ImmutableMap.<String, Vocab> of("",
-      AltStylesheetVocab.VOCAB);
+      AggregateVocab.of(AltStylesheetVocab.VOCAB, new UncheckedVocab("", "")));
   private static Map<String, Vocab> KNOWN_VOCAB_URIS = ImmutableMap.of(MagazineNavigationVocab.URI,
       MagazineNavigationVocab.VOCAB, ForeignVocabs.PRISM_URI, ForeignVocabs.PRISM_VOCAB);
   private static Set<String> DEFAULT_VOCAB_URIS = ImmutableSet.of(StructureVocab.URI);

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

@@ -27,7 +27,7 @@ CSS_001=The "%1$s" property must not be included in an EPUB Style Sheet.
 CSS_002=Empty or NULL reference found.
 CSS_003=CSS document is encoded in UTF-16. It should be encoded in UTF-8 instead.
 CSS_004=CSS documents must be encoded in UTF-8, detected %1%s;
-CSS_005=Conflicting alternate style attributes found: %1$s.
+CSS_005=Conflicting alternate style tags found: %1$s.
 CSS_006=CSS selector specifies fixed position.
 CSS_007=Font-face reference "%1$s" refers to non-standard font type "%2$s".
 CSS_008=An error occurred while parsing the CSS: %1$s.

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

@@ -367,14 +367,23 @@ Feature: EPUB 3 — Content Documents — XHTML
   ####  Links
 
   Scenario: Verify a `link` element with a known alt style tag
-    When checking document 'link-alt-style-tags-valid.xhtml'
+    Given the reporting level is set to usage
+    When checking document 'link-alt-style-tags-known-valid.xhtml'
+    Then no usages are reported
+    And no errors or warnings are reported
+
+  Scenario: Report a `link` element with an unknown alt style tag
+    Given the reporting level is set to usage
+    When checking document 'link-alt-style-tags-unknown-valid.xhtml'
     Then no errors or warnings are reported
+    And no usages are reported
 
   Scenario: Report a `link` element with an unknown alt style tag
-    When checking document 'link-alt-style-tags-error.xhtml'
-    Then error OPF-027 is reported
-    And error CSS-005 is reported
-    And no other errors or warnings are reported
+    Given the reporting level is set to usage
+    When checking document 'link-alt-style-tags-conflict-usage.xhtml'
+    Then no errors or warnings are reported
+    And usage CSS-005 is reported
+    But no other usages are reported
 
   Scenario: Verify `link` elements used to declare alternative stylesheets
     When checking document 'link-rel-stylesheet-alternate-valid.xhtml'

src/test/resources/epub3/06-content-document/files/link-alt-style-tags-conflict-usage.xhtml

@@ -0,0 +1,9 @@
+<!DOCTYPE html>
+<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
+	<head>
+		<meta charset="utf-8" />
+		<title>alt style tag css declaration</title>
+		<link rel="stylesheet" href="horizontal.css" class="day night" />
+	</head>
+	<body></body>
+</html>

src/test/resources/epub3/06-content-document/files/link-alt-style-tags-error.xhtml → src/test/resources/epub3/06-content-document/files/link-alt-style-tags-unknown-valid.xhtml

@@ -2,8 +2,8 @@
 <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
 	<head>
 		<meta charset="utf-8" />
-		<title>invalid alt style tag css declaration</title>
-		<link rel="stylesheet" href="horizontal.css" class="horizontal invalid vertical" />
+		<title>unknown alt style tag css declaration</title>
+		<link rel="stylesheet" href="horizontal.css" class="unknown" />
 	</head>
 	<body> </body>
 </html>