use XML syntax highlighting if provided language is html

web/react/utils/markdown.jsx

@@ -106,14 +106,20 @@ class MattermostMarkdownRenderer extends marked.Renderer {
     }
 
     code(code, language) {
-        if (!language || highlightJs.listLanguages().indexOf(language) < 0) {
-            let parsed = super.code(code, language);
+        let usedLanguage = language;
+
+        if (String(usedLanguage).toLocaleLowerCase() === 'html') {
+            usedLanguage = 'xml';
+        }
+
+        if (!usedLanguage || highlightJs.listLanguages().indexOf(usedLanguage) < 0) {
+            let parsed = super.code(code, usedLanguage);
             return '<div class="post-body--code"><code class="hljs">' + TextFormatting.sanitizeHtml($(parsed).text()) + '</code></div>';
         }
 
-        let parsed = highlightJs.highlight(language, code);
+        let parsed = highlightJs.highlight(usedLanguage, code);
         return '<div class="post-body--code">' +
-            '<span class="post-body--code__language">' + HighlightedLanguages[language] + '</span>' +
+            '<span class="post-body--code__language">' + HighlightedLanguages[usedLanguage] + '</span>' +
             '<code class="hljs">' + parsed.value + '</code>' +
             '</div>';
     }