The web is open since the very beginning defined by standard whatever the terminal or operating system moreover the web can adapt to the size of the terminal with a relative values which means that you use percentage when giving the size of an element instead of fixed value. Openness and relativity are the keys of web developments
The advantage of designing for multiple platforms are numerous. The end user has the same visibility whatever the supporting technology and features are the same on each platforms with only one solution to push in production but it's more expensive with an unknown return of investment.
Above all the survival of your internet business rely on the experience of the end user knowing that an internet solution can be extremely complex and drive the end user to your competitor. It's then absolutely necessary to take into account the experience of the end user during the development or test of your solution.
You then have to choose between :
- traditional website for computer
- Mobile first website
- Responsive Design Website which adapt to the platform
- two solutions, one for computers and one for mobiles with .htaccess router.