In which way to Google and Alexa determine the rank of a site?
I have an account on Alexa, but it doesn't show any information because my site visitors are few (500-600 per day).
Google PageRank is always 0.
I followed all standards of responsive design (for mobile and desktop), good keywords, w3c validations, cool articles, and backlinks. Why does my website have the same rank as a plain white page would?
Alexa Rank is based on visitors and backlinks and it's not consider On-Page SEO.
Google Rank is calculated on both Off-Page and On-Page SEO (responsive design, page titles, descriptions, content and validations are relevant).
However, google rank is very slow to increase (in my experience); if your site is online from less than 1 year, don't worry.