This is a very bad practice to teach new programmers

Or, maybe, everyone learns a little differently and their approach is useful for most, or it might just be your opinion. Please show some code examples to have a reasonable discussion and understand the point you are trying to make?

DIrectly from the video:

// index.html
...
<div class="primary-content t-border rounded">
...

// style.css
...
.primary-content {
  background: grey;
}
.t-border {
  border-top: 1px solid;
}
.rounded {
  border-radius: 10px;
}
...

Is presented as be better than:

// index.html
...
<div class="primary-content">
...

// style.css
...
.primary-content {
  background: grey;
  border-top: 1px solid;
  border-radius: 10px;
}
...

Which simply isn't true, and is not beneficial for new programmers to be concerned with. It is much easier to search and replace for objects, and fix the ones you missed after doing testing, rather than handcuffing objects to one another for fear of breaking the nonsense rule that you should not repeat yourself ever.

You can teach classes without accidentally teaching tight coupling.

To the new people - be as expressive and clear as you want to be, and don't do things that can feel awkward clunky just because you believe there is some golden rule that cannot be broken. Code is meant to be read (and debugged) by humans, not by computers, and in my example it is clear what is going on, versus the example given as an example of optimization. If you wrote my code 6 months ago, worked on 10 different projects, then came back to it, you would be able to jump back in much faster than the code given as "better" in the video. Once you've got a few projects under your belt, the reason for creating general-use classes will become much clearer, but until then, keep those fingers typing and gaining valuable muscle memory instead of compulsively refactoring snippets into classes.

*edit

I forgot a crucial part to my point - the naming of things. New programmers are horrible at naming things by nature, because life hasn't taught us to think that way before. Just like learning to play an instrument or writing a song, it's gonna be painful at first. The less things you have to name, the less likely you are going to name something poorly and create bugs/flawed decisions down the line on the project. More classes, more naming.

Here is a nice post that beginners to CSS and programming in general should take a look at (not mine, I swear): https://medium.com/swlh/excellent-code-clean-and-beautiful-code-b541ca4b5a39