Increasing testing requirements on what? I meant adding it to the browser (or the window manager/X/whatever), which would not require a massive effort to test.

Increasing testing requirements on what? I meant adding it to the browser (or the window manager/X/whatever), which would not require a massive effort to test. The websites themselves already have to support (at least) three modes of interaction if they want to have all main browsers supported (mouse, touch, keyboard).