outline:1px solid red;

dabblet.css

body {
    background:url(http://i.imgur.com/qKsVr.png);
}

header {
    width:240px;
    background:	white;
	margin:50px; 
	  
}

header:before {
	content:"   ";
    display:block;
    position: relative;
    width: 240px;
    bottom:30px;
    height:30px;
/*    outline:1px solid red; */
    background: linear-gradient(50deg, transparent 75%, white 75%) 0 50%,
                linear-gradient(-50deg, transparent 75%, white 75%) 0 50%;
    background-repeat: repeat-x;
    background-size:30px 30px, 30px 30px;
    -moz-transform: rotate(180deg);
    -webkit-transform: rotate(180deg);
    -ms-transform: rotate(180deg);
    -o-transform: rotate(180deg);
    transform: rotate(180deg);

}

header:after {
    content: " ";
    display:block;
    position: relative;
    width: 240px;
    bottom:-30px;
    height:30px;
/*    outline:1px solid red; */
    background: linear-gradient(-50deg, transparent 75%, white 75%) 0 50%,
                linear-gradient( 50deg, transparent 75%, white 75%) 0 50%;
    background-repeat: repeat-x;
    background-size:30px 30px, 30px 30px;
}

	

dabblet.html

<header>Lorem ipsum dolor sit amet .. jk. No Latin here. Filler text extends and stretches this element box.</header>

settings.json

{"view":"split","fontsize":"100","seethrough":"1","prefixfree":"1","page":"css"}