JSFiddle - React, Tailwind, and code Playground

HTML

<div contenteditable="true" id="selector" style="background-color: rgb(255,255,0);">
    Test back<span style="background-color: rgb(255,0,0);">ground</span> color.
</div>

JavaScript

document.designMode = "on";

var selectionParent = function () {
    var parent = null, sel;

    if (window.getSelection) {
        sel = window.getSelection()
        if (sel.rangeCount) {
            parent = sel.getRangeAt(0).commonAncestorContainer
            if (parent.nodeType != 1) {
                parent = parent.parentNode
            }
        }
    } else if ( (sel = document.selection) && sel.type != "Control") {
        parent = sel.createRange().parentElement()
    }
    
    return parent
}

$('div#selector').on('mouseup', function (){
    alert($(selectionParent()).css('background-color'));
});