JSFiddle - React, Tailwind, and code Playground

HTML

<div id="test">
    Lorem <span class="highlighted">ipsum</span> dolor sit amet, <span class="highlighted">consectetur</span> <span class="highlighted">adipiscing</span> elit. Sed massa.
<div>

JavaScript

function combineSpansIn(selector, spanClass) {
    // initialize precedent values
    var prec=null;
    var precNodeType;

    $(selector).contents().each(function(){
        if ($.trim( $(this).text() ) !== "") { // empty nodes will be useless here
            var nodeType = this.nodeType;

            // check if still a combinable span
            if (nodeType == 1 && this.className==spanClass && nodeType == precNodeType) {
                // append current node to precedent one
                $(prec).append(" "+ $(this).text() );
                
                // remove current node
                $(this).remove();
            } else {
                // update precedent values
                prec=this;
                precNodeType = nodeType;
            }
        }
    });
}

combineSpansIn('#test', 'highlighted');