But there doesn’t appear to be a simple way to test for :placeholder-shown.

You can’t use @supports for selectors, only property/values (e.g. @supports (display: flex))

This creates an options object with a getter function for the passive property; the getter sets a flag, passiveSupported, to true if it gets called. That means that if the browser checks the value of the passive property on the options object, passiveSupported will be set to true; otherwise, it will remain false. We then call addEventListener() to set up a fake event handler, specifying those options, so that the options will be checked if the browser recognizes an object as the third parameter.

Perhaps we should detect URLSearchParams objects differently (using duck typing detection instead of instanceof window.URLSearchParams, for example) but the solution isn't adding a specific polyfill to Axios (as it'd increase the bundle size and still won't work with other polyfills).

Sometimes we can’t implement a solution that’s fully spec-compliant, and in those cases using a polyfill might be the wrong answer. A polyfill would translate into telling the rest of the codebase that it’s okay to use the feature, that it’ll work just like in modern browsers, but it might not in edge cases.