Can one represent a web application without a domain but without relying on IP addresses solely?
No - I cannot see that this is possible. IP addresses are fundamental to the use of the internet. Domain names allow some abstraction away from the direct use of the IP address. Without the domain name, you're back to using the IP address directly.
Is there ...
I recommend Visual Studio Code (VSCode) with the Live Server extension. It works best with Google Chrome. VSCode is a free download from Microsoft.
Since you're working with CSS, I would also recommend the IntelliSense for CSS class names in HTML extension.
You could also try something like codepen.io or codesandbox.io. They both allow you to edit HTML &...
The HTTP specification says so in section 3.2.2:
If the port is empty or not given, port 80 is assumed.
This is something that has been a part of HTTP since it was invented by Tim Berners-Lee. He chose the default port and allowed URLs to omit :80 to let it be the default. This was done presumably to make URLs cleaner.
HTTPS has a similar default ...