In \Xoops\Form\Select (XoopsCore/htdocs/xoops_lib/Xoops/Form/Select.php) setting the selected attribute uses selected="selected" however per HTML standard should just be selected...

in renderOption() method change lines 104-106 from:

if (in_array($optionValue, $selected)) {
    $rendered .= ' selected="selected"';
}

to:

if (in_array($optionValue, $selected)) {
    $rendered .= ' selected';
}

This effects multiple XoopsForm Objects since several classes use \Xoops\Form\Select. Same issue exists in XOOPS 2.5.x series - I'll open a separate issue there too...