Why CSS is not working in PDF created by Visualforce ?

In Many cases, i have observed problems faced by my colleagues and complaining that CSS is not working when they try to render any Visualforce page as “PDF”. Same Question is asked many times in Interviews also. Basically there are two ways:

1. Use “apex:stylesheet” tag to import external CSS file
2. Wrap “Style” tag inside “Head” tag in Visualforce