Recently I've seen a lot of tutorials on editing in Fandom, but noticed one difference: they use automatically CSS editor. Is there a way to switch to CSS editor? Is there a way to edit the page with HTML? If so, how?
What tutorials? What do you mean by "automatically"? The CSS editor should be used on CSS pages when you have syntax highlighting enabled. If you have disabled syntax highlighting, then it won't be used. As for HTML, pages are created in wikitext; not HTML. Wikitext allows for some HTML but there are limits. See Help:HTML.