Prepends class / id selectors with "ns_" if data-namespace="ns" is given on the <link> tag.

nsify.js

(function() {

var nsify = function(ns, selChunk) {
  sels = selChunk.replace(/ /g, '').split(',');

  for(var i = 0; i < sels.length; ++i)
    if(sels[i][0].match(/[.#]/))
      sels[i] = sels[i][0] + ns + '-' + sels[i].substring(1);

  return sels.join(',');
};

var cssText = '';

for(var i = 0; i < document.styleSheets.length; ++i) {
  var sheet = document.styleSheets[i];

  if(!sheet.ownerNode.dataset)
    continue;
  if(!sheet.ownerNode.dataset.namespace)
    continue;

  var ns = sheet.ownerNode.dataset.namespace;

  sheet.disabled = true;
  for(var j = 0; j < sheet.cssRules.length; ++j) {
    var rule = sheet.cssRules[j];
    var newsel = nsify(ns, rule.selectorText);

    cssText += rule.cssText.replace(/^(.*?){/, newsel + ' {') + '\n\n';
  }
}

var style = document.createElement('style');
style.type = 'text/css';
if(style.styleSheet)
  style.styleSheet.cssText = cssText;
else
  style.appendChild(document.createTextNode(cssText));

document.getElementsByTagName('head')[0].appendChild(style);

})();