code: function walkAttrs(html, fn) {
    if (!html)
      { return }
    var m;
    while (m = RE_HTML_ATTRS.exec(html))
      { fn(m[1].toLowerCase(), m[2] || m[3] || m[4]); }
  }
docs: Minimize risk: only zero or one _space_ between attr & value
  @param   { String }   html - html string we want to parse
  @param   { Function } fn - callback function to apply on any attribute found