function safeAttrValue(tag, name, value, cssFilter) {
  // unescape attribute value firstly
  value = friendlyAttrValue(value);

  if (name === "href" || name === "src") {
    // filter `href` and `src` attribute
    // only allow the value that starts with `http://` | `https://` | `mailto:` | `/` | `#`
    value = _.trim(value);
    if (value === "#") return "#";
    if (
      !(
        value.substr(0, 7) === "http://" ||
        value.substr(0, 8) === "https://" ||
        value.substr(0, 7) === "mailto:" ||
        value.substr(0, 4) === "tel:" ||
        value.substr(0, 11) === "data:image/" ||
        value.substr(0, 6) === "ftp://" ||
        value.substr(0, 2) === "./" ||
        value.substr(0, 3) === "../" ||
        value[0] === "#" ||
        value[0] === "/"
      )
    ) {
      return "";
    }
  } else if (name === "background") {
    // filter `background` attribute (maybe no use)
    // `javascript:`
    REGEXP_DEFAULT_ON_TAG_ATTR_4.lastIndex = 0;
    if (REGEXP_DEFAULT_ON_TAG_ATTR_4.test(value)) {
      return "";
    }
  } else if (name === "style") {
    // `expression()`
    REGEXP_DEFAULT_ON_TAG_ATTR_7.lastIndex = 0;
    if (REGEXP_DEFAULT_ON_TAG_ATTR_7.test(value)) {
      return "";
    }
    // `url()`
    REGEXP_DEFAULT_ON_TAG_ATTR_8.lastIndex = 0;
    if (REGEXP_DEFAULT_ON_TAG_ATTR_8.test(value)) {
      REGEXP_DEFAULT_ON_TAG_ATTR_4.lastIndex = 0;
      if (REGEXP_DEFAULT_ON_TAG_ATTR_4.test(value)) {
        return "";
      }
    }
    if (cssFilter !== false) {
      cssFilter = cssFilter || defaultCSSFilter;
      value = cssFilter.process(value);
    }
  }

  // escape `<>"` before returns
  value = escapeAttrValue(value);
  return value;
}
default safeAttrValue function

@param {String} tag
@param {String} name
@param {String} value
@param {Object} cssFilter
@return {String}