This doesn't make CSS classes mutable global state. CSS rules are applied according to specificity of selectors. You could maybe say that the overall style of a

This doesn't make CSS classes mutable global state. CSS rules are applied according to specificity of selectors. You could maybe say that the overall style of a page is mutable state, but not the CSS classes themselves. A CSS class is assigned and that's it. Unless JS is involved, it also doesn't disappear or toggle.