posthtml-inline-css 
PostHTML plugin for inlining CSS to style attrs.
Many modern email clients nowadays support CSS in a <style>, so you might not need to inline CSS.
See that discussion.
Usage
Plain CSS
var posthtml = require('posthtml'),
css = 'div { color: red }';
posthtml([require('posthtml-inline-css')(css)])
.process('<div style="font-size: 14px">Hello!</div>')
.then(function (result) {
console.log(result.html);
});
// <div style="font-size: 14px; color: red">Hello!</div><style>
var posthtml = require('posthtml'),
html = '<style>div { color: red }</style><div>Hello!</div>';
posthtml([require('posthtml-inline-css')()])
.process(html)
.then(function (result) {
console.log(result.html);
});
// <style>div { color: red }</style><div style="color: red">Hello!</div>PostCSS
var posthtml = require('posthtml'),
postcss = require('postcss'),
postcssObj = postcss(/* some PostCSS plugins */).process('div { color: white }');
posthtml([require('posthtml-inline-css')(postcssObj)])
.process('<div style="font-size: 14px">Hello!</div>')
.then(function (result) {
console.log(result.html);
});
// <div style="font-size: 14px; color: white">Hello!</div>