CSS-in-JS is yet another form of colocation used by frameworks like React which use JavaScript render functions instead of templates. As Evan Czaplicki, Elm’s creator, posited in Twitter ...

CSS-in-JS refers to a pattern where CSS rules are produced through JavaScript instead of defined in external CSS files. Two sub-patterns coexist.

The new fetch and render tool warns you when you block CSS and JavaScript. We also know, Google renders the page as a user would see it these days, so blocking CSS/JS can impact that big time.