Word for a web site that can be used on mobile/tablet/PC/others
You could go with multiplatform.
Compatible with or involving more than one type of computer or operating system. [Oxford]
Responsive Web Design is the use of HTML and CSS to automatically tailor the display of the website to the device that it is being viewed on.
A common term is Portable.
Portable software is a software program that can be run on different computers running different operating systems with no or little modifications.
Usually portable is used to describe how software is written. Software that is written portably will use functions that are universal or standardized (e.g. POSIX). A portable application can run on many systems. When non-portable software needs to run on another system, it must be re-written, or ported. Software that is already portable works out of the box or with minimal modifications
If it is easy to get to run on another system, it is portable, otherwise it is said to be non-portable.
A similar term is cross-platform.
A cross-platform computer product or system is a product or system that can work across multiple types of platforms or operating environments. Different kinds of cross-platform systems include both hardware and software systems, as well as systems that involve separate builds for each platform, as well as other broader systems that are designed to work the same way across multiple platforms.
While portable describes how an individual program is written, cross-platform simply describes the end result. Cross-platform software may actually be multiple different versions of the software, tailored to each individual operating system. This is not portable, but it is cross-platform because it runs on more than one system. Cross-platform software may require the developers modify it, often heavily, in order to allow it to run on another system. A video game may be cross-platform if it runs on both the PS4 and Xbox One, but it is not portable as it is not written to be system-agnostic.
If it runs on multiple systems, it is cross-platform, otherwise it is single-platform.