Add explanation why lang takes precendence over xml:lang

xemlock · Aug 19, 2020 · dbe79de · dbe79de
1 parent e1acc1c
commit dbe79de
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Showing 2 changed files with 8 additions and 4 deletions.
diff --git a/library/HTMLPurifier/AttrTransform/HTML5/Lang.php b/library/HTMLPurifier/AttrTransform/HTML5/Lang.php
@@ -24,13 +24,17 @@ public function transform($attr, $config, $context)
             $attr['lang'] = $lang = $xml_lang;
         }
 
-        // If both lang and xml:lang are present, ensure they're in sync,
-        // lang having priority over xml:lang
+        // If lang and xml:lang are both present, ensure that their values are
+        // equal, with lang having priority over xml:lang, as according to the
+        // spec, lang attribute in the XML namespace is not allowed on HTML
+        // elements, and non-namespaced attribute with the literal localname
+        // 'xml:lang' has no effect on language processing.
+        // https://html.spec.whatwg.org/#the-lang-and-xml:lang-attributes
         if ($lang !== false && $xml_lang !== false && $xml_lang !== $lang) {
             $attr['xml:lang'] = $lang;
         }
 
-        // xml:lang will be stripped out unless %HTML.XHTML is true
+        // xml:lang will be stripped out later unless %HTML.XHTML is true
 
         return $attr;
     }

diff --git a/tests/HTMLPurifier/HTMLModule/HTML5/CommonAttributesTest.php b/tests/HTMLPurifier/HTMLModule/HTML5/CommonAttributesTest.php
@@ -21,7 +21,7 @@ public function testXmlLang()
         // When the attribute xml:lang in no namespace is specified, the element must also have the attribute lang present with the same value.
         $this->assertPurification('<p xml:lang="en">text</p>', '<p xml:lang="en" lang="en">text</p>');
 
-        // When the attribute xml:lang in no namespace is specified, the element must also have the attribute lang present with the same value.
+        // When there is a conflict between xml:lang and lang values, lang takes precedence
         $this->assertPurification('<p lang="en" xml:lang="pl">text</p>', '<p lang="en" xml:lang="en">text</p>');
 
         $this->assertPurification('<hr xml:lang="en">', '<hr xml:lang="en" lang="en" />');