Loading html from file using JavaScript
I have some issues with your code and possible alternatives for implementation.

Usage of var

In Javascript using var instead of let or const are somewhat discouraged (in the new era of ES6). There is some value in block scope for loops using var but you can certainly always use ES6' let instead. Since you are not reassigning xmlHttp you might as well const that (same goes for x and let for url).

Use fetch instead of XMLHttpRequest

Fetch comes with a promise-ready easier API for handling requests. CanIUse reports that it's almost supported on every new Browser as well. A polyfill should be enough to support the rest of the browsers out there. It also helps in readability and simplicity.

In your case, getting HTML out of a requests is a simple as:

fetch(...)
  .then(function (response) {
    // Status 200? Type text/html? Might aswell check that here.
    return response.text();
  })
  .then(function (body) {
    // Insert into dom here...
    console.debug(body);
  })


Error handling thanks to fetch

What happens if your URL is faulty, temporarily unavailable and some other error? If you need error handling then just add a catch below then:

.catch(e){
  // Display message, console silenty
  console.debug(e);
}


Using external libraries is usually not necessary as ES6 brings the most bang for the buck anyway.