Why “Mobile First” May Already Be Outdated

"Mobile First" describes a methodology of starting on smaller screens with less 'estate' such that no important details are left out.

"Desktop First" makes it really easy for your solution to something looking bad to be "just display:none it", which can make the mobile version feel like an afterthought (because it often is).

"Mobile First" just means you take into consideration the mobile aspects and make sure everything important is included - and you scale it to "Desktop sizes" afterwards, since you no longer need to hide anything (only change how it is displayed).

The design, regardless if Mobile or Desktop "first", should look good on any screen. That's what Responsive Design is about.

You can do Mobile-First Responsive Design. The two aren't separated and the article seems to pretend that they are. Responsive Design has always been about making sure the product looks good regardless of screen size.

Because that can be expensive for complicated sites - many web design companies choose common screen sizes like the iPhone, popular Android series (Galaxy/Note), and common Desktop Resolutions. The best designers do away with all of those constraints and design based on "when does this design look bad?" rather than arbitrary break points.