Several ways to call styles using scripts_javascript skills

I often encountered words like classname and csstext before, but I didn’t know how to use them. I recently started to actually build a website and gained a lot. I would like to share it with those who don’t understand yet.

There are usually four ways to call style sheets in web pages. The first is external links, that is, in the form of ; the second is to enter the style sheet; the third is to declare it at the head of the web page, such as