content-mode: append for attributes?

[cj-clx]

Should this work? Or does content-mode: append; only work for element content, not attributes?

require "vendor/autoload.php";

$xml = <<<XML
<input value=""/>
XML;

$tss = <<<TSS
input:attr(value) {
    content: "first_part";
}
input:attr(value) {
    content: "_suffix";
    content-mode: append;
}
TSS;

$data = [];

$template = new Transphporm\Builder($xml, $tss);
echo $template->output($data)->body . PHP_EOL;