Combine all web page source into single index.html?

Input: an index.html with many external .js and .css files; no images.

Desired output: a new html file ('index_combined.html') that contains ALL the source.

How does one go about doing this in an automated fashion?

(The only definite way I can think of is using PhantomJS to render the page to html, but that's a bit heavyweight and has side effects.)