Which do you prefer, developing mobile first or desktop first?
Is it better to design for the desktop then shrink down to mobile?
Is it easier to start at the mobile size then expand to desktop mode?
What are the pros and cons of each?
How small should we go for mobile screens?
How big should we design for large monitors?
most web content today is consumed with mobile devices so it just makes sense to develop mobile first. whether or not your app is mobile accessible should depend on your target audience.
responsive design is standard practice. once upon a time people called it mobile first. its easy to make your site responsive now adays with css frameworks like bootstrap, foundation and others
the advantage of responsive design is that you dont have to try to add messy code to retrofit your site if you decide to make it mobile later on which makes it simpler to grow and maintain.
i dont think desktop first is really anything other than not putting time into responsive design. maybe if you just need to get something up fast then i guess, but your all your divs should be responsive regardless.
use something like screenfly to test how your site looks on different screens. then its just a matter of tweaking a few styles depending on the screen size.