W3.CSS is a free and modern CSS framework published by W3school for developing responsive, mobile-first websites.
It is smaller and faster than similar CSS framework (Bootstrap). It is easier to learn and use.
It simply speedup the development of website.
W3.CSS is inspired from Google Material Design.
W3.CSS helps in responsive designing of website which means designing of website in such a way that the contents of websites automatically fit and resize that makes the website looks good on all devices (PC, laptop, tablets or mobile phones) wherever it is opened.
Now a days, users view the website using different devices (PC, laptop, tablets or
mobile phones) and the screen size of the devices differs from one another, therefore
the designing of the website must be in such a way that it looks good in all the devices.