Responsive web design (RWD) makes web pages render well on different devices by using fluid grids and layouts, flexible images and media, and CSS media queries. The viewport meta tag allows controlling page dimensions and scaling. RWD uses a grid system divided into columns, box-sizing to include padding and borders, and media queries with CSS properties for certain screen widths. It follows a mobile-first approach where mobile styles are set as default and overridden by larger screens. Images and videos are made responsive using width:100% and max-width:100%.

1. Responsive Web Development
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2. Responsive
Web
Development:
Responsive web design is an approach to web design that
makes web pages render well on a variety of devices and
window or screen sizes from minimum to maximum display
size.
Recent work also considers the viewer proximity as part of
the viewing context as an extension for RWD

4. RWD Viewport:
The viewport is the user's visible area of a web page.
The viewport varies with the device, and will be smaller
on a mobile phone than on a computer screen.
Before tablets and mobile phones, web pages were
designed only for computer screens, and it was
common for web pages to have a static design and a
fixed size.

5. Setting RWD Viewport:
HTML5 introduced a method to let web designers take
control over the viewport, through the <meta> tag. You
should include the following <meta> viewport element
in all your web pages:
<meta name="viewport" content="width=device-width,
initial-scale=1.0">
This gives the browser instructions on how to control
the page's dimensions and scaling.

7. RWD Grid View:
Many web pages are based on a grid-view, which
means that the page is divided into columns:

8. Building A Responsive Grid-View:
Lets start building a responsive grid-view.
First ensure that all HTML elements have the box-sizing
property set to border-box. This makes sure that the
padding and border are included in the total width and
height of the elements.
* {
box-sizing: border-box;
}

9. RWD Media Queries:
Media query is a CSS technique introduced in CSS3. It
uses the @media rule to include a block of CSS
properties only if a certain condition is true.

10. Always Design For Mobile First:
Mobile First means designing for mobile before
designing for desktop or any other device (This will
make the page display faster on smaller devices).
This means that we must make some changes in our
CSS. Instead of changing styles when the width gets
smaller than 768px, we should change the design when
the width gets larger than 768px.

12. RWD Images:
If the width property is set to a percentage and the
height property is set to "auto", the image will be
responsive and scale up and down:

13. Max Property RWD
Images:
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:

14. Different Images For
Different Devices:
A large image can be perfect on a big computer screen,
but useless on a small device. Why load a large image
when you have to scale it down anyway? To reduce the
load, or for any other reasons, you can use media
queries to display different images on different devices.

16. RWD Videos:
If the width property is set to 100%, the video player will
be responsive and scale up and down:

17. Max Property RWD
Videos:
If the max-width property is set to 100%, the video
player will scale down if it has to, but never scale up to
be larger than its original size: