How to add such a triangle to the page?

Question

css

Can you please tell me how to add such a triangle to the corner of the page?

Answer 1

Or you draw a block with a pseudo-class before and transform

Answer 2

Based on the screenshot, little is clear.
I can make an assumption that this is just an image on the site and the triangle was simply drawn, and the text is already superimposed on top

Answer 3

Or you create an Xpx x Xpx image with this triangle and paste it in the upper left corner