I have a large css file in use. Have experienced several times that new added css rules where not applied. After long searching for a minor mistake or after placing the new part earlier in the css file I got it working.
That made me wondering: What is the best way to check large css file for parts breaking up the css?  Is there some tool for checking css for breaking parts?
 

w3c css validator

https://jigsaw.w3.org/css-validator/

you can paste your css directly or can paste css link there and fix the errors one by one that show up there

Ah great thanks.

Do you know if there is  a plugin for browsers (FF or Chrime)  to only show the media querys ? to be able to work on responsive for mobile
Now I use the responsive tool but that is not always clear since it is all mixed up.

I think most CSS is actually written as Sass/Scss/Less etc these days and then compiled down to CSS through an automatic build tool like webpack, so if there's any errors then it won't compile down and they would be an error in the console.

For viewing the website as responsive, you can just use the F12 dev tools built into most modern browsers, it will let you display the website at a particular size, and let you inspect the style rules applied at that size, toggle them, edit them in the browser to experiment with what they look like etc.

I can't imagine doing any form or web dev or designing without writing something like Scss, having a build process and error checking, and using browser dev tools to tweak and inspect a page