好问题，我自己问的，下面是我想到的。您可以在自定义插件或functions.php.

// turn on Output Buffering (hence *ob*)
ob_start();
// register a callback to run after Wordpress has outputed everything
add_action(\'shutdown\', function () { 
    // get the output buffer and store it to a variable
    $html = ob_get_clean();
    // setup the PHP native html beautifier called tidy
    // note that you need to have libtidy installed
    // but chances are that your server already have it
    $tidy = new \\tidy;
    $tidy->parseString($html, [
        \'indent\' => true,
        \'output-xhtml\' => true,
        \'wrap\' => 200
    ], \'utf8\');
    $tidy->cleanRepair();
    // output the beautified html
    echo $tidy;
    // Use 0 as third parameter to the add_action so you will
    // register the action as early as possible. Otherwise 
    //  Wordpress will flush the output buffer before you do...
}, 0);