Link css file after main content is loaded and parsed

Is there any way to link to an external css file but to download and parse it only when the main part of the website have been already loaded?

It is the case of the footer of a website, where its css shouldn't delay the load of the main sections of the page. I know I can inline css code in the HTML code, but in order to have the code organized I prefere not to do it. Is the only way to do this to use a php include?

Same question for Javascript.