For CSS you can still inspect elements with devtools, fortunately, and that will show you the style properties, even if the styles are set programmatically.
I'd say not too much from a high perspective. Browser support for new HTML5/CSS3 specs got better, so it's safer to use stuff like flexboxes now. But resources for learning the basics from then should still be relevant, and potentially got updated anyway.
Regarding JS, there's an ongoing shift of hype between various front end frameworks like React, Angular or Vue. Others go back to vanilla JS, as it has better standardization and more power nowadays. But maybe that's not too relevant for you.
CSS Grid is now available and constitutes a major change in how layout can (should) be done. Of course there should be fallbacks so older browsers can still get a page that can be used (not pixel perfect).
Writing JavaScript using newer ES5, ES6 features and transpiling it has become more common.
I haven't coded a page in years. So, pardon my ignorance, but is Bootstrap really required?
I ask because I too am marginally interested in creating something. Though, I suspect I'll just cheat and use a CMS like Wordpress and a bog-standard theme.
As someone looking to learn web design for my personal site too, from what I can see Bootstrap and similar frameworks aren't really needed provided you're willing to learn all the tech utilised by them and do it from scratch yourself, either for learning purposes or because you prefer to avoid frameworks for some other reason.
The frameworks exist for professional web designers who don't want to redo the same stuff over and over, and want code with maximum browser compatibility and covering edge cases, bullet-proof and so on - not goals that you would probably prioritise when just starting out learning or creating hobbyist sites.
I'm considering using a CMS or a WYSIWYG editor too, but the prospect of creating a minimal, semantically rich and modern site in HTML5/CSS3, is too tempting to not spend the time learning the ropes. Besides, efficient use of these CMS/editors themselves take fair bit of effort, which we might as well employ to learn the foundations, or so I feel. :-)
As tempting as it is, the project I have in mind will be time consuming, so the devoted time is probably best spent generating content - in my case.
Like you, I am sorely tempted to undertake an in-depth learning. Years ago, I wrote a comprehensive HTML tutorial. It covered the entire spec. Of course, that was v. 2.0, so it's hardly valid today.
It touched on CSS, the tutorial, but I'm not sure it was even a complete standard back then. I think it was just a single page in my tutorial.