Minimalism is seen as having few or less of something i.e possessions and keeping things simple and clean cut. Notice those bolded words? well they are what i see as the key to minimalistic web design.
Simple and clean cut
Now back end simple is very much meaning formatted code (online formatters or PHP storm), comments only one line big no commented out code that may or may not be used later. Use CDN’s for your CSS and JS files again keep these files minified and only use them IF possible. For page speed ensure if you do have to use jquery or js that these are loaded in last so that page load speed is not compromised. Lets Encrypt free SSL certificates…. please get one at least, a green padlock and SECURE in the browser bar is a nice touch plus it had security benefits of course. Lastly your domain name, as short as possible but still relevant of course to your cause or branding. Also ensure that it gets indexed as https://domainname.tld not https://www.domainname.tld as we seemed to have moved away from www meaning worldwide web and its just not commonly used anymore.
Having few or less of something
Now come some key parts where possible have less of the following, this increase page load time and UX:
You can still have images of course or an iframe video but only if relevant and will make of break the content of that site.
A good minimalistic website will load in under 2 seconds, be fully responsive on all devices and screen widths, be fully functional, easy to navigate and most of all be fun.