...
Some vendors have extensions to C, and some also have implemented only part of the C standard before stopping development. Consequently, it is not possible to back up and only discuss C95, or C90. The vendor support equation is too complicated to draw a line and say that a certain compiler supports exactly a certain standard. Whatever demarcation point is selected, different vendors are be on opposite sides of it for different parts of the language. Supporting all possibilities would require testing the cross product of each compiler with each language feature. Consequently we have selected a demarcation point that is the most recent in time, so that the rules and recommendations defined by the standard will be applicable for as long as possible.
...