W3Schools Tryit Editor

<!DOCTYPE html>
<html>
<script>
function includeHTML() {
  var z, i, elmnt, file, xhttp;
  /*loop through a collection of all HTML elements:*/
  z = document.getElementsByTagName("*");
  for (i = 0; i < z.length; i++) {
    elmnt = z[i];
    /*search for elements with a certain atrribute:*/
    file = elmnt.getAttribute("w3-include-html");
    if (file) {
      /*make an HTTP request using the attribute value as the file name:*/
      xhttp = new XMLHttpRequest();
      xhttp.onreadystatechange = function() {
        if (this.readyState == 4) {
          if (this.status == 200) {elmnt.innerHTML = this.responseText;}
          if (this.status == 404) {elmnt.innerHTML = "Page not found.";}
          /*remove the attribute, and call this function once more:*/
          elmnt.removeAttribute("w3-include-html");
          includeHTML();
        }
      }      
      xhttp.open("GET", file, true);
      xhttp.send();
      /*exit the function:*/
      return;
    }
  }
};
</script>
<body>

</body>
</html>

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<!DOCTYPE html><html><script>function includeHTML() {  var z, i, elmnt, file, xhttp;  /*loop through a collection of all HTML elements:*/  z = document.getElementsByTagName("*");  for (i = 0; i < z.length; i++) {    elmnt = z[i];    /*search for elements with a certain atrribute:*/    file = elmnt.getAttribute("w3-include-html");    if (file) {      /*make an HTTP request using the attribute value as the file name:*/      xhttp = new XMLHttpRequest();      xhttp.onreadystatechange = function() {        if (this.readyState == 4) {          if (this.status == 200) {elmnt.innerHTML = this.responseText;}          if (this.status == 404) {elmnt.innerHTML = "Page not found.";}          /*remove the attribute, and call this function once more:*/          elmnt.removeAttribute("w3-include-html");          includeHTML();        }      }            xhttp.open("GET", file, true);      xhttp.send();      /*exit the function:*/      return;    }  }};</script><body>​<div w3-include-html="h1.html"></div> <div w3-include-html="content.html"></div> ​<script>includeHTML();</script>​</body></html>​