如何以编程方式更改CSS以限制其范围？

I've got a chunk of CSS as a string, and I want to insert it into the DOM such that it only applies to elements in a particular container. Some tools, like Polymer, for example, rewrite CSS selectors so they only apply within a limited scope. How can I do something similar so that, when I insert this CSS into the DOM, it won't change all elements on the page?I've got a chunk of CSS as a string, and I want