we have different screen sizes and
resolution and browsers load
images before css and js.so if i
have a standard-image screen(low
resolution) i should not wait for
the page to load and waste
bandwidth.browsers need somebody
to tell them wether this screen is
a Retina 4K or standard-resolution
.add on that the screen size
-smartphone with retina screen or
standard low resolution screen
.HTML <code>srcset</code> and
<code>sizes</code> do the job.
