Comparing XAMPP and php -S localhost:8000, they look very different

I've set up local testing using both XAMPP and php -S localhost:8000. When I compare the website that is produced from both, the website behaves and looks different from each other. However, the same codebase hosted on the web server looks completely fine and behaves alright.

How am I supposed to know if my changes will work properly on the actual web server if they don't show up properly during local testing?

I'm forced to test locally because my co-workers don't seem to know how to set up the dev environment for me to edit the files on a staging server accurately.... Please help!