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HTML

<!-- CSS3 Shapes – kształty za pomocą CSS3 – część 1 - przykład 26 -->
<!-- http://danielpietrasik.pl/css3-shapes-ksztalty-za-pomoca-css3/ -->

<div id="example26"></div>

CSS

#example26 {
	width: 0;
	border: 12px solid transparent;
	border-bottom: 10px solid blue;
	position: relative;
	top: -10px;
}
#example26:after {
	content: '';
	position: absolute;
	left: -5px; 
    top: 10px;
	width: 0;
	border: 5px solid transparent;
	border-top: 20px solid green;
    border-bottom: 0px;
}