How is min-width media query rule s mobile first approach - html

Question

They always say that min-width #media rule is the way to build for mobile first, I have read plenty articles about it but i still can't understand how exactly min-width rule works> But the max-width is easy and lends itself to easy comprehension.
#media only screen and (min-width: 400px) {....some rule here.....}
#media only screen and(min-width: 900px){......some rule here....}
my question and confusion is: can one used both breakpoint on the same stylesheets? and how does it make for mobile first ?
I need a tolerable responses please, no down voting for those who enjoy down voting please be tolerable and nice enough to help put.

Answer 1

Indeed its true using min-width helps to make a web mobile first.
Let us take an example.
We are creating a web that will scale to two viewports say 300px, 300px+ devices.
1) using min-width
body {
background: yellow;
}
// 300px+ devices
#media (min-width: 300px) {
body {
background: red;
}
}
Here background-color is been overridden for 300px+ devices
2) using max-width
body {
background: red;
}
// 300px- devices
#media (max-width: 300px) {
body {
background: yellow;
}
}
Here background-color is been overridden for 300px- devices
Now down the line in your App timeline you need to support 600px+ devices
3) using min-width
body {
background: yellow;
}
// 300px - 600px devices
#media (min-width: 300px) {
body {
background: red;
}
}
// 600px+ devices
#media (min-width: 600px) {
body {
background: green;
}
}
New media query added to support 600+ devices, no changes needed in the existing style sheet.
4) using max-width
body {
background: green;
}
// 600px- devices
#media (max-width: 600px) {
body {
background: red;
}
}
// 300px- devices
#media (max-width: 300px) {
body {
background: yellow;
}
}
Although we needed additional media-query rule to support 600+ devices, but we needed to change the global body background-color to support new breakpoint.
Now compare 1) with 3) and 2) with 4) ,
you will notice to support new breakpoint
for 1 to 3 we didn't need to change existing style rules, just added new rules over it.
but for 2 to 4 existing rules were modified to support new breakpoint
Summary
so min-width ensures future friendly and progressive enhancement (mobile-first)
but max-width leds to short-sighted approach and needs degradation (mobile-last)