Hi Sebastian,
It's difficult to tell without a link to the site or more details, but I suspect you are either a) trying to reference a path outside the 'public' directory, or b) using relative paths to URLs instead of absolute ones whilst using rewrites. Let's say your project structure looks like this (and assuming you selected the default 'public' directory when you ran `firebase init`):
/your/project/directory
- firebase.json
- outside/
- main.css
- public/
- index.html
- inside/
- main.css
With regard to a) - one thing that will work if you just open public/index.html directly on your machine that won't if you use `firebase serve` on localhost, is the link:
<style type="text/css" href="../outside/main.css" />
Instead you should use (you cannot break out of the public directory):
<style type="text/css" href="/inside/main.css" />
Secondly, regarding b) if you're creating a single page app and using rewrites, if you visit localhost:5000/some/deep/path and have a relative link like this:
<style type="text/css" href="inside/main.css" />
It will try and look for /some/deep/path/inside/main.css instead of /inside/main.css
If that doesn't cover your issue, feel free to provide a link to your deployed site or more specific information
Chris
Engineer @ Firebase