What's the width to mock-up websites for desktop and mobile now??

If you want your website to be prepared for use in the wide, wild world, you should keep anyscreen width between, say, and px in mind. Modern designers for responsivewebdesigns (designs that adapt to the width of the viewport) work with ranges of widths. Each range has its own quirks to adapt the website to the width.

Yaqoub Alomar is right.. I stongly agree with his answer.