content: Error message for unexpected CSS class in vlist inner span

Author: rajveermalviya

lib/model/katex.dart

@@ -195,13 +195,17 @@ class _KatexParser {
             for (final innerSpan in vlist.nodes) {
               if (innerSpan case dom.Element(
                 localName: 'span',
-                className: '',
                 nodes: [
                   dom.Element(localName: 'span', className: 'pstrut') &&
                       final pstrutSpan,
                   ...final otherSpans,
                 ],
               )) {
+                if (innerSpan.className != '') {
+                  throw KatexHtmlParseError('unexpected CSS class for '
+                    'vlist inner span: ${innerSpan.className}');
+                }
+
                 var styles = _parseSpanInlineStyles(innerSpan)!;
                 final topEm = styles.topEm ?? 0;