function minify_html(){
    ob_start('html_compress');
}

function html_compress($buffer){

    $search = array(
        '/n/',         // replace end of line by a space
        '/>[^S ]+/s',     // strip whitespaces after tags, except space
        '/[^S ]+</s',     // strip whitespaces before tags, except space
        '/(s)+/s',     // shorten multiple whitespace sequences,
        '~<!--//(.*?)-->~s' //html comments
    );

    $replace = array(
        ' ',
        '>',
        '<',
        '\1',
        ''
    );

    $buffer = preg_replace($search, $replace, $buffer);

    return $buffer;
}

add_action('wp_loaded','minify_html');