The grammar for the vector-effect property currently only allows one keyword. (| combinator is an exclusive OR in the CSS values grammar).

Value: none | non-scaling-stroke | non-scaling-size | non-rotation | fixed-position

However, the prose says:

These values can be enumerated. Thereby, the effect which has these characteristics simultaneously can be specified.

Logically, it is reasonable to apply multiple effects at once. The grammar should be changed to use the || combinator, except for the none keyword:

Value: none | [ non-scaling-stroke || non-scaling-size || non-rotation || fixed-position ]

I do not expect there will be any no implementation impacts of making this change, since there are currently no implementations that implement more than one keyword value.

cc @satakagi