[abhishekcpp007]

The classes check in check_url_schemes joins its two conditions with and, so it only fires when the value is not a list and has a non-string item. That never happens in practice:

• "classes": "external-gh" passes, and the link renders as class="e x t e r n a l - g h"
• "classes": [1, 2] passes, then the HTML writer crashes with AttributeError: 'int' object has no attribute 'startswith'

Switched it to or. Added a parametrized test in TestUrlSchemes for a string, a list with an int, and a bare int, through both MdParserConfig and the topmatter merge, plus one that a list of str still passes.