Question

Is there any way to create a sharp flat corner with CSS and HTML?

Something like this:

  ____
 /    \
 |    |
 \____/

Answer 1

Look here. There you find all you need:

http://css-tricks.com/examples/ShapesOfCSS/

Edit
In case the link goes lost:

CSS

#octagon { 
  width: 100px; 
  height: 100px; 
  background: red;  
  position: relative; 
} 

#octagon:before { 
  content: ""; 
  position: absolute;  
  top: 0; left: 0; 
  border-bottom: 29px solid red; 
  border-left: 29px solid #eee; 
  border-right: 29px solid #eee; 
  width: 42px; height: 0; 
} 

#octagon:after { 
  content: ""; 
  position: absolute; 
  bottom: 0; 
  left: 0; 
  border-top: 29px solid red; 
  border-left: 29px solid #eee; 
  border-right: 29px solid #eee; 
  width: 42px; 
  height: 0; 
}