Edit in JSFiddle

var range = document.createRange();
var p1 = document.getElementById("p1");

var helloNode = p1.firstChild.firstChild;
var worldNode = p1.lastChild;
// 범위 설정
range.setStart(helloNode, 2); 
range.setEnd(worldNode,3);

// 새 노드 생성
var newNode = document.createElement("span");
var newText = document.createTextNode("삽입된 텍스트");
newNode.appendChild(newText);
newNode.style.color = "orange";

range.insertNode(newNode); // 범위에 삽입

<p id="p1"><b>Hello</b> world!</p>