public function process()
        {
            if ($this->_isXhtml === null) {
                $this->_isXhtml = (false !== strpos($this->_html, '<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML'));
            }

            $this->_replacementHash = 'MINIFYHTML' . md5($_SERVER['REQUEST_TIME']);
            $this->_placeholders = array();

            // replace SCRIPTs (and minify) with placeholders
            $this->_html = preg_replace_callback(
                    '/(\\s*)(<script\\b[^>]*?>)([\\s\\S]*?)<\\/script>(\\s*)/i'
                    , array($this, '_removeScriptCB')
                    , $this->_html);

            // replace STYLEs (and minify) with placeholders
            $this->_html = preg_replace_callback(
                    '/\\s*(<style\\b[^>]*?>)([\\s\\S]*?)<\\/style>\\s*/i'
                    , array($this, '_removeStyleCB')
                    , $this->_html);

            // remove HTML comments (not containing IE conditional comments).
            $this->_html = preg_replace_callback(
                    '/<!--([\\s\\S]*?)-->/'
                    , array($this, '_commentCB')
                    , $this->_html);

            // replace PREs with placeholders
            $this->_html = preg_replace_callback('/\\s*(<pre\\b[^>]*?>[\\s\\S]*?<\\/pre>)\\s*/i'
                    , array($this, '_removePreCB')
                    , $this->_html);

            // replace TEXTAREAs with placeholders
            $this->_html = preg_replace_callback(
                    '/\\s*(<textarea\\b[^>]*?>[\\s\\S]*?<\\/textarea>)\\s*/i'
                    , array($this, '_removeTextareaCB')
                    , $this->_html);

            // trim each line.
            // @todo take into account attribute values that span multiple lines.
            $this->_html = preg_replace('/^\\s+|\\s+$/m', '', $this->_html);

            // remove ws around block/undisplayed elements
            $this->_html = preg_replace('/\\s+(<\\/?(?:area|base(?:font)?|blockquote|body'
                    . '|caption|center|cite|col(?:group)?|dd|dir|div|dl|dt|fieldset|form'
                    . '|frame(?:set)?|h[1-6]|head|hr|html|legend|li|link|map|menu|meta'
                    . '|ol|opt(?:group|ion)|p|param|t(?:able|body|head|d|h||r|foot|itle)'
                    . '|ul)\\b[^>]*>)/i', '$1', $this->_html);

            // remove ws outside of all elements
            $this->_html = preg_replace_callback(
                    '/>([^<]+)</'
                    , array($this, '_outsideTagCB')
                    , $this->_html);

            // use newlines before 1st attribute in open tags (to limit line lengths)
            $this->_html = preg_replace('/(<[a-z\\-]+)\\s+([^>]+>)/i', "$1\n$2", $this->_html);

            // fill placeholders
            $this->_html = str_replace(
                    array_keys($this->_placeholders)
                    , array_values($this->_placeholders)
                    , $this->_html
            );

            return $this->_html;
        }