JSFiddle - React, Tailwind, and code Playground

HTML

<p id="textSection">الَلَهُم صَلِ عَلى مُحَمّد و آل مُحمد</p>
<button id="highlight" onclick="highlight();">Highlight</button>

CSS

.highlighted {
    
    background-color:yellow;
}

JavaScript

function stripDiacritics(rawString) {
   return rawString.replace(/ُ|ِ|َ|ٍ|ً|ّ/g,"");        
}

function highlight() {
    var textblock = document.getElementById('textSection').innerHTML;
    var words = textblock.split(' ');
    var resultText = '';
    
    for (var i=0; i < words.length; i++) {
        var strippedWord = stripDiacritics(words[i]);
        if (strippedWord == 'محمد') {
            resultText += ' <span class="highlighted">'+words[i]+'</span>';
        }            
        else {
            resultText += ' '+words[i];
        }
    }
        
    document.getElementById('textSection').innerHTML = resultText;
    
}