Edit in JSFiddle

.sibling-element {
	background: #f2f2f2;
	padding: 10px;
  border: 1px solid #81adc8;
} 

.main-element {
    position: fixed;
    bottom: 10px;
    left: 10px;
    background-color: yellow;
    padding: 10px;
}

.parent-element {
    position: relative;
    height: 100px;
    padding: 10px;
    background-color: #81adc8;
}

html {
    height: 800px;
}