What is Responsive Web Design?
Responsive Web Design is about using HTML and CSS to automatically resize, hide, shrink, or enlarge, a website, to make it look good on all devices (desktops, tablets, and phones):

Setting The Viewport
To create a responsive website, add the following <meta> tag to all your web pages:
Example
<meta name="viewport" content="width=device-width, initial-scale=1.0">

This will set the viewport of your page, which will give the browser instructions on how to control the page's dimensions and scaling.

Responsive Images
Responsive images are images that scale nicely to fit any browser size.

Using the width Property
If the CSS width property is set to 100%, the image will be responsive and scale up and down:
example:-
image.html file :-
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>

<h2>Responsive Image</h2>
<p>When the CSS width property is set in a percentage value, the image will scale up and down when resizing the browser window. Resize the browser window to see the effect.</p>

<img src="img_girl.jpg" style="width:100%;">

</body>
</html>

Notice that in the example above, the image can be scaled up to be larger than its original size. A better solution, in many cases, will be to use the max-width property instead.

Using the max-width Property
If the max-width property is set to 100%, the image will scale down if it has to, but never scale up to be larger than its original size:

solution with max-width :-
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>

<h2>Responsive Image</h2>
<p>"max-width:100%" prevents the image from getting bigger than its original size. However, if you make the browser window smaller, the image will still scale down.</p>
<p>Resize the browser window to see the effect.</p>

<img src="img_girl.jpg" style="max-width:100%;height:auto;">

</body>
</html>

Responsive Text Size
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:
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>

<h1 style="font-size:10vw;">Responsive Text</h1>

<p style="font-size:5vw;">Resize the browser window to see how the text size scales.</p>

<p style="font-size:5vw;">Use the "vw" unit when sizing the text. 10vw will set the size to 10% of the viewport width.</p>

<p>Viewport is the browser window size. 1vw = 1% of viewport width. If the viewport is 50cm wide, 1vw is 0.5cm.</p>

</body>
</html>

Media Queries
In addition to resize text and images, it is also common to use media queries in responsive web pages.

With media queries you can define completely different styles for different browser sizes.

Example: resize the browser window to see that the three div elements below will display horizontally on large screens and stack vertically on small screens:

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<style>
* {
  box-sizing: border-box;
}

.left {
  background-color: #2196F3;
  padding: 20px;
  float: left;
  width: 20%; /* The width is 20%, by default */
}

.main {
  background-color: #f1f1f1;
  padding: 20px;
  float: left;
  width: 60%; /* The width is 60%, by default */
}

.right {
  background-color: #04AA6D;
  padding: 20px;
  float: left;
  width: 20%; /* The width is 20%, by default */
}

/* Use a media query to add a break point at 800px: */
@media screen and (max-width: 800px) {
  .left, .main, .right {
    width: 100%; /* The width is 100%, when the viewport is 800px or smaller */
  }
}
</style>
</head>
<body>

<h2>Media Queries</h2>
<p>Resize the browser window.</p>

<p>Make sure you reach the breakpoint at 800px when resizing this frame.</p>

<div class="left">
  <p>Left Menu</p>
</div>

<div class="main">
  <p>Main Content</p>
</div>

<div class="right">
  <p>Right Content</p>
</div>

</body>
</html>