The best CSS and Javascript unmnifier is Firefox

There are a lot of unminify services and tools out there. But you don't need any of them, because Firefox (or Chrome/Chromium/Brave/Vivaldi/Edge, etc.) already has a built-in unminifier.

Here's the way to do it in Firefox:

Open your Developer Tools by hitting CTRL+SHIFT+I and go to the Debugger tab. Under the Sources tab, you'll see all of the Javascript files organised in directories. You can double-click any of them and then click the { } button in the bottom left corner of the file for "Pretty Print". This will add whitespace to make the file readable (i.e. unminify it). That's all there is to it. If there's a Source Map, the file will be unminified by default.

You can do the same thing for CSS files by going to the Style Editor. It also has a "Pretty Print" option.

And that's it! Most unminify services and tools don't do anything more than this. Your browser can already do this perfectly well.

In some cases, you may need to disable Sourcemaps before you can enable Pretty Print for some sources. This is because the file links to a sourcemap that the server hasn't served, so you need to tell Firefox to ignore the sourcemap.