[DHPersonal]

The consensus that I believe has been reached in the front-end community is to design the CSS with mobile versions as the default — no media queries — and then adjust the design with cascading media queries to manage specific needs of the various sizes that grow from the default. With grid and flex offering truly responsive layouts without excessive or restrictive media queries, adjusting for tablet and desktop screens consists largely of filling in any layout rendering/browser quirks or handling design quibbles.

The reasoning for this is fairly simple: a majority of users visit the site on mobile devices. Sites are being developed with a single code base for the largest group of visitors, with tablet and desktop users being added on afterward almost as alternative views. Using this process doesn't always produce the best results for any browser size because it requires a design plan that considers the requirements of this method.

[jfengel]

Even if a majority of users aren't on mobile, there will be at least some. And mobile renders better on a desktop than desktop renders on a mobile.

Desktop-only sizes and features are great add-ons if you have time. But being able to get MVP to 100% of users is the top priority.