I think juice is what you’re looking for.
Simply require it, then pass it your html and css and let it do the heavy lifting for you like this:
var juice = require('juice');
var inlinedcss = juice('<p>Test</p>', 'p { color: red; }');
It builds on a number of mature libraries including mootools’ slick, and supports a broad range of selectors.
You may also be interested in node-email-templates, which is a nice wrapper for dynamic emails in node.