How access localhost on high Sierra

Are you asking about running a web server? If so you open a browser and type localhost in the address bar. If you have the web server running you will see the default page. If you have not set up your web server yet then follow these instructions:

https://coolestguidesontheplanet.com/install-apache-mysql-php-and-phpmyadmin-on- macos-high-sierra-10-13/

Another approach from the Terminal:

1. Change directory to the folder containing your web site, or web page (e.g. index.html).
2. python -m SimpleHTTPServer [nnnn] # for python 2.7.n
   1. [nnnn] is optional and defaults to port 8000
   2. Usage: python -m SimpleHTTPServer 8080
3. python -m http.server [nnnn] # for python 3 only

Now in a new browser window/tab, assuming that you used the default port 8000:

http://localhost:8000

will automatically show the index.html content in the above folder. Or you can display a specific web page:

http://localhost:8000/foo.html

Have fun.

FWIW, I suggest sticking with a more ubiquitous server that is better supported.

Exactly the words that I should have included with the Python example. Not a replacement for the AMP stack, or server-side action.