The
ControlRenderingCompatibilityVersion setting[
^] simply activates a different code path in the .NET 4.0 code, which forces certain controls to render in the same obsolete way that they did in v3.5, ignoring the new improvements introduced in v4.
It doesn't introduce a dependency on v3.5, and your site will continue to work on a computer that doesn't have v3.5 installed.
However, you should evaluate why you need that setting, and look to modify your CSS and Javascript so that it works without that setting. The new rendering mode is much cleaner and more standards-compliant.