I’ve edited your post for readability. When you enter a code block into a forum post, please precede it with a separate line of three backticks and follow it with a separate line of three backticks to make it easier to read.
You can also use the “preformatted text” tool in the editor (</>) to add backticks around text.
I’m not really sure what you’re trying to do, but if the plan is to create a shape by using border-properties on an element with zero width, I don’t think that’s possible with border-image.