Please add "required" tags to joomlaLogin_vertical.php and joomlaLogin_horizontal.php.
placeholder="<?php echo JText::_('MOD_SCLOGIN_USERNAME'); ?>">
to
placeholder="<?php echo JText::_('MOD_SCLOGIN_USERNAME'); ?>" required aria-required="true">
and
placeholder="<?php echo JText::_('MOD_SCLOGIN_PASSWORD') ?>">
to
placeholder="<?php echo JText::_('MOD_SCLOGIN_PASSWORD') ?>" required aria-required="true">