These plugins are written in ES6, and intended to be used in modern browsers (Chrome, Safari, Firefox, Edge) without transpilation. Many of these plugins make use of CSS Custom Properties (CSS variables) for functionality, so any browser that doesn’t support these features will have trouble with.
As far as I am aware, the only browser that supports CSS Custom Properties but not the ES6 features used (where transpiling to ES5 might improve support) is for Mobile Safari (iOS) support.
Currently all of these plugins are under active development and things are shifting around quite a bit so browser support may change with each release.
For 20+ CSSplus demos, check out the CSSPlus demos collection on Codepen.