Keyword densityy is the percentage of times a keyword or phrase appears on a web page compared to the total number of words on the page.
The reason why you haven't had a reply to this is because the answer is no, they don't have a reference or link because such information doesn't currently exist, i.e. there is no "proper keyword density".
Keyword density is the percentage of times a keyword is found in your page content among all the other content. For example, if you have a Web page with exactly 100 words on it including all headlines, captions, alt text, and advertising, and you have a keyword phrase that is on the page 4 times, your keyword density is 4%.