That's a part of Responsive Web Design

Responsive web design is about creating web pages that look good on all devices!
A responsive web design will automatically adjust for different screen sizes and viewports.

To create a responsive website, add the following "meta" tag to all your web pages:

Why this is important?

And imagine can also be resonsive,
With the css attribute "max-width:", you can try to size the website windows and see the "walking man" below size change.

The text size can be set with a "vw" unit, which means the "viewport width".
That way the text size will follow the size of the browser window

There are also responisve CSS file to download, they offer the frame work for putting content.

Now the website design are also towards energy saving, that's why it's better if a designer know how to make a picture quicklier downloaded.