How to remove the border part, like on the layout?

Question

css

I came across such a border in the layout and I don’t know how to make it

Answer 1

Pseudo-element version:
https://codepen.io/abnmt/pen/yKOzOM
Linear-gradient version:
https://codepen.io/abnmt/pen/KozXmW

Answer 2

Variant times

div{position:relative;border:2px solid black;border-bottom:none;}
div:after{}//левая сторона
div:before{}//правая сторона

Option 2

div{position:relative;border:2px solid black;border-bottom:none;}
div:after{background:linear-gradient;}

Answer 3

Similar question with solution:
How to cut a border to paste text over it?

Answer 4

won't the legend tag be enough?
htmlbook.ru/html/legend