JSFiddle - React, Tailwind, and code Playground

HTML

<table id="seiten" width="100%"  border="0">
  <tr>
    <td style="width:25px" width="25"><input class="itm_checkbox" value="true" type="checkbox" name="seiten[0][aktiv]" checked="checked" /></td>
    <td><input class="itm_input" type="text" style="width: 98%" name="seiten[0][value]" /></td>
    <td><img src="http://findicons.com/files/icons/1156/fugue/16/plus_button.png" onclick="addRow('seiten', this)" /><img src="http://findicons.com/files/icons/1156/fugue/16/minus_button.png" onclick="deleteRow('seiten', this)" /></td>
  </tr>
</table>

JavaScript

function addRow(tableID, r) {
	var table = document.getElementById(tableID);
	var currentRow = r.parentNode.parentNode;
	var newRow = currentRow.cloneNode(true);
	newRow.getElementsByTagName("input")[1].value = "";
	table.tBodies[0].insertBefore(newRow,currentRow.nextSibling);
	reNumberRows(tableID);
}

function deleteRow(tableID, r) {
    var table = document.getElementById(tableID);
	table.tBodies[0].removeChild(r.parentNode.parentNode);
	reNumberRows(tableID);
}

function reNumberRows(tableID){
	var table = document.getElementById(tableID);
	var checkboxes = table.getElementsByClassName('itm_checkbox');
	var inputs = table.getElementsByClassName('itm_input');
	for(i=0;i<checkboxes.length;i++){
		checkboxes[i].name = "seiten[" + i + "][aktiv]";	
	}
	for(i=0;i<inputs.length;i++){
		inputs[i].name = "seiten[" + i + "][value]";	
	}
}