function get_font_size()
    {

        if ($this->__font_size_calculated) {
            return $this->_props["font_size"];
        }

        if (!isset($this->_props["font_size"])) {
            $fs = self::$_defaults["font_size"];
        } else {
            $fs = $this->_props["font_size"];
        }

        if (!isset($this->_parent_font_size)) {
            $this->_parent_font_size = self::$default_font_size;
        }

        switch ((string)$fs) {
            case "xx-small":
            case "x-small":
            case "small":
            case "medium":
            case "large":
            case "x-large":
            case "xx-large":
                $fs = self::$default_font_size * self::$font_size_keywords[$fs];
                break;

            case "smaller":
                $fs = 8 / 9 * $this->_parent_font_size;
                break;

            case "larger":
                $fs = 6 / 5 * $this->_parent_font_size;
                break;

            default:
                break;
        }

        // Ensure relative sizes resolve to something
        if (($i = mb_strpos($fs, "em")) !== false) {
            $fs = (float)mb_substr($fs, 0, $i) * $this->_parent_font_size;
        } else if (($i = mb_strpos($fs, "ex")) !== false) {
            $fs = (float)mb_substr($fs, 0, $i) * $this->_parent_font_size;
        } else {
            $fs = (float)$this->length_in_pt($fs);
        }

        //see __set and __get, on all assignments clear cache!
        $this->_prop_cache["font_size"] = null;
        $this->_props["font_size"] = $fs;
        $this->__font_size_calculated = true;
        return $this->_props["font_size"];

    }