code: function _traverseRules(sheet, base) {
              var i,
                  href = sheet.href,
                  cssRules;
  
              // Deal with Firefox's SecurityError when accessing sheets
              // from other domains. Chrome will safely return `undefined`.
              try {
                  cssRules = sheet.cssRules;
              } catch (e) {
                  if (e.name !== "SecurityError") {
                      throw e;
                  }
              }
  
              if (href && cssRules) {
                  if (rel.stylesheets[href] === undefined) {
                      rel.stylesheets[href] = [];
                  }
                  rel.stylesheets[href].push(base);
  
                  for (i = 0; i < cssRules.length; i++) {
                      if (cssRules[i].href) {
                          traverseRules(cssRules[i].styleSheet, base);
                      }
                  }
              }
          }
docs: traverse @import rules