Once you've got hold of the text nodes containing the text you want highlighted, it's easy. How you get those depends on how generic you need this to be. As it is at the moment, before the user has edited it, the following will work:
var mainDiv = document.getElementById("main");
var startNode = mainDiv.firstChild.firstChild;
var endNode = mainDiv.childNodes[2].firstChild;
var range = document.createRange();
range.setStart(startNode, 6); // 6 is the offset of "world" within "Hello world"
range.setEnd(endNode, 7); // 7 is the length of "this is"
var sel = window.getSelection();
sel.removeAllRanges();
sel.addRange(range);
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