Of course this question doesn't have a simple answer, but there are some metrics and tools we can use to measure with.
First it is good to note that computers have become so powerful that websites are not limited to the public facing information displays we are familiar with. Facebook, Aggie.io, and Jitsi Meet are all built with largely the same technologies. So websites are also capable of providing powerful functionality.
This article focuses on the quality website internals. How efficiently is it built and runs. Website desgin has many aspects, far beyond the scope of this article.
Today people access the internet from all sorts of devices, mostly from phones. websites have the advantage of being so widely accepted across all the different computers, this is not a small thing to be overlooked, it has taken alot of work and consultation to achieve.
The website should work well for some one using it on a desktop with a mouse and keyboard, and equally well for someone on a small phone using only one thumb to navigate. Generally this means a visitor should not have to zoom in or pan around to read text and view content.
To accomplish this the website has a layout that responds to the screen dimensions, hence responsive design.
We also don't want the buttons to be hard to press with fingers, which tend to be somewhat less accurate than mouse clicks.
The website should load and start as quickly as possible. We are all growing accustomed to smooth load times and if a website takes too long to get together we will often decide "Oh, I guess this website doesn't work...", and find another one.
"Oh, guess this website doesn't work..."
There are plenty of studies about the effects of a website's load time and speed on conversion rates and use.
It's alright if you know what you're trading a bit of speed or loadtime for, but it better be valuable, because that speed is.
You're website should be structured properly so that it can be easily navigated by a screen reader so that it can be effectively used by the blind, low vision, or folks with other obstacles.
Unless you are blessed with a blind mother-in-law, this may be unfamiliar territory. Make sure it's not to the team building your websites and mobile applications.
Search Engine Optimization
You may come across this written SEO. Simply, it means making your website easy to navigate for search engines like duckduckgo. If the search engines find what they need they can provide an accurate summary of your website.
These optimizations include providing the structured meta data, as well as all the other things we have talked about. They all influence whether search engines will provide your website on their list, and how far from the top you will be.
Website Audit Tools
You may not be able to jump under the hood of your website and understand what you see, or how to measure it. And for a quick look over, I won't do that either. Before digging into the code of a site to identify strengths and pain points, use an audit tool to highlight where they might be.
The industry standard right now is a tool called Google Lighthouse. While this tool requires a bit of knowledge to work, Google has a site set up called PageSpeed Insights which you can run for a quick lookover of your site.
It uses Google Lighthouse under the hood, so the results will be consistent, and it will give you a good idea of how you are doing.
Hopefully this gives you some starting ground, and a tool to work with in managing your web presence. Don't get overwhelmed, and you're always welcome to reach out to Ridván for a consultation on how you can move forward.