You can, but you’re using it in a different way than you would on a full-custom site.
Back in the days of Adobe Dreamweaver, people crafted their web sites from HTML using an HTML-aware text editor. It could edit Javascript and CSS too. It was possible, and indeed necessary, to write custom HTML for everything, or to cut-n-paste it from someplace. Microsoft FrontPage went a little further, helping its users write the HTML. But it still posted HTML to the web site. This kind of web site is often called “static”.
WordPress, and many other content management systems, on the other hand, use the custon HTML provided by their owners as a template. They keep the conttent separate from the HTML. When a user wants to see the content, the CMS “pours” it into the template HTML.
Let me explain this situation with an example.
I just finished updating a static, full custom HTML / CSS web site full of lyrics of songs for a well-known local folk musician. The site has existed since 1997 or something like that, but he had some newer albums that weren’t in there. https://lyrics.gordonbok.com/ (If you’re into maritime folk ballads about storms and lobster pots and all that, you might like his music.) Full custom HTML.
Now, I strongly considered scraping the old site and building another one in WordPress, to publish the same material. I could have done that. I’d create posts for song lyrics, posts for albums, and maybe posts for guest artists, and all that. I’d figure out how to implement nav links like next-song, whole-album, and all that, maybe with categories, tags, and ordering. And WordPress would generate the hyperlinks and menus and navigation and indexes and all that in the HTML. I still would have implemented a mess of CSS (either in a baby plugin or in Additional CSS in the theme or subtheme).
There would have been plenty of custom CSS and HTML in the project done this way: but it would have been done separately from the content (Gordon’s lyrics).
Why didn’t I do that? Because it would have taken too long, to build a WordPress structure around the lyrics, before I could have changed anything visibe to site visitors. It was easier to add the new material to the old Dreamweaver site. If I were starting a new song-lyrics site I would definitely use WordPress.
Now, if you’re going to publish a full custom HTML site, WordPress doesn’t add much value. You still have to hack away at the HTML to change anything, you can’t just change a post or two.
I hope this clarifies the distinction between static-site custom HTML and WordPress custom HTML