Unable to draw a triangle. How to do it?

Question

HTML

I can’t draw a triangle, I don’t understand what is wrong HERE . I would be very grateful for your help!

Answer 1

There are generators for creating triangles for a long time: apps.eky.hk/css-triangle-generator

Answer 2

The :before / :after pseudo-elements must have the property specified content: '', otherwise it will not be rendered.
You have exactly this case - add to :aftercontent: ''

Answer 3

As already answered above, you don't have the content: ''. And this is one of the most nasty mistakes) I
recommend reading an extremely informative guide to pseudo-classes and pseudo-elements , it is translated into Russian.