function highlight(parent, startoff, endoff) {
  // Erm...
  parent.textContent;
}

// Test cases

var starts = [
  5,  44, 0, 50, 6,  100, 99,  50, 51, 52
];
var ends = [
  20, 62, 4, 70, 10, 100, 101, 54, 53, 53
];
for (var i = 0; i < 10; i += 1) {
  highlight(document.getElementById("target"),
            starts[i], ends[i]);
}

#target {
  white-space: pre-wrap;
}

<span id="target">
'Twas brillig, and the slithy toves
  Did gyre and gimble in the wabe:
All mimsy were the borogoves,
  And the mome raths outgrabe.

"Beware the Jabberwock, my son!
  The jaws that bite, the claws that catch!
Beware the Jubjub bird, and shun
  The frumious Bandersnatch!"

He took his vorpal sword in hand:
  Long time the manxome foe he sought --
So rested he by the Tumtum tree,
  And stood awhile in thought.

And, as in uffish thought he stood,
  The Jabberwock, with eyes of flame,
Came whiffling through the tulgey wood,
  And burbled as it came!

One, two! One, two! And through and through
  The vorpal blade went snicker-snack!
He left it dead, and with its head
  He went galumphing back.

"And, has thou slain the Jabberwock?
  Come to my arms, my beamish boy!
O frabjous day! Callooh! Callay!'
  He chortled in his joy.

'Twas brillig, and the slithy toves
  Did gyre and gimble in the wabe;
All mimsy were the borogoves,
  And the mome raths outgrabe.
</span>

let starts = [
    5,  44, 0, 50, 6,  100, 99,  50, 51, 52
];
let ends = [
    20, 62, 4, 70, 10, 100, 101, 54, 53, 53
];

let positions = [];
let normalizedPositions = [];
starts.forEach(function(position) {
    positions.push({position, value: 1});
});
ends.forEach(function(position) {
    positions.push({position, value: -1});
});
positions = positions.sort(function(a, b) {
    return a.position - b.position || 
        b.value - a.value
});

var currentSection = {from: 0, counter: 0};

for(position of positions) {
    if (!currentSection.counter) {
        if (position.value === -1) {
            throw `inconsistent boundaries: closing before opening ${position.position}`;
        }
        currentSection.from = position.position;  
    }
    currentSection.counter += position.value;

    if (!currentSection.counter) { 
        normalizedPositions.push({
            from: currentSection.from, 
            to: position.position
        });
    }
}
if (currentSection.counter) {
    throw "last section has not been closed properly";   
}


let parentNode = document.querySelector('p');
let textNodeToReplace = parentNode.childNodes[0];
let sourceText = textNodeToReplace.nodeValue;

let documentFragment = document.createDocumentFragment();
let withoutHighlightingStart = 0;

normalizedPositions.forEach(function (highlightRange) {
    if (highlightRange.from> withoutHighlightingStart) {
      let notHighlighted = createTextNode(sourceText.slice(withoutHighlightingStart, highlightRange.from));
      documentFragment.appendChild(notHighlighted);
    }
    let highlighted = createHighlighted(sourceText.slice(highlightRange.from, highlightRange.to));
    documentFragment.appendChild(highlighted);
    withoutHighlightingStart = highlightRange.to;
});
let lastNotHighlighted = createTextNode(sourceText.slice(withoutHighlightingStart));
documentFragment.appendChild(lastNotHighlighted);

parentNode.replaceChild(documentFragment, textNodeToReplace);

function createTextNode(str) {
   return document.createTextNode(str);
}

function createHighlighted(str) {
   let span = document.createElement('span');
   span.classList.add('highlight');
   span.appendChild(createTextNode(str));
   return span;
}

.highlight {
    background-color: yellow;
    color: dark-blue;
}

<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>