demo purposes

dabblet.css

* {
	box-sizing: border-box; /* www.paulirish.com/2012/box-sizing-border-box-ftw/‎ */
	}
	

.page {
	display: flex;
	}
	
/* demo purposes */
.welcome, .main {height: 200px; width: 300px;}
.welcome {
	background-color: dodgerblue;
	}
.main {
	background-color: pink;
	}

/* resize window to see it in action */
@media (max-width:500px) {
	.page {
		flex-direction: column-reverse;
		}
	}

dabblet.html

<div class="page">
    <div class="welcome">
        <span>Welcome to my page</span>
    </div>
    <div class="main">
        <div class="header">
        </div>
        <div class="content">
        </div>
    </div>
</div>

settings.json

{"view":"split-vertical","fontsize":"100","seethrough":"","prefixfree":"1","page":"all"}