As long as neocities, nekoweb and other sites alike exist HTML will always be a free way to share your thoughts across space. When you compare that to payment locked services like notion pages, or even the rather cheap https://getupnote.com/ or https://home.omg.lol/sign-in. Sure the user-interface(UI) is a bit offputting when you compare so the dazzling and easy to use strawpage and wordpress but the freedom is very much worth it. Plus over time you will defintely get use to the neocities clouds text editor.
w3schools is a great place to learn the fudementals. It includes almost everything you need to know. I think it missing proper explainations on semantic markup and media queries so my recommendation is to watch YouTube videos on these topics.
1) Most retro and neocities sites are not mobile compatiable, or are designed for specific screensizes.
A very easy way to fix this is by using media queries. This essentially means when the screen is a certain size the webpage acts differently.
You could hide elements, such as large images, change font-size or you could even zoom out by default.
2) Not using image compression/dithers. Feel like this one is quite self-explanatory so won't expand further that this.
3) Lacking content. The heart of personal sites are super super long passages of text, displaying artwork/photography
and expression of your love for your interests whether that's by joining fanlisting or building shrines.
4) Rememebering to removed EXIF/geolocation from photos before uploading.
There are many softwares you can download these days to do this. Find one that works for you.
5) Not adding a robot.txt page: if you don't want your website to be crawled and archived, then this is a must.
6) Choosing the right guessbook. There have been countless cases where someone has written something uncalled for,
this is a hard one as most guestbooks don't come with moderation features (where you can block certain words for example).
I would say at least pick one that allows IP blocking.