You don’t want to display all your errors for the world to see so when you create web page you will want to check that they actually display correctly before you upload them to the web.
Testing Server Side Processing Before you Upload

Often a web page takes far longer to load than it needs to. Many web beginners forget to properly optimise the parts of their web page. A properly optimised web page might look identical to one that hasn’t been optimised but may be only a small fraction of the total size and therefore will be [...]