css - Change text color to white on any non-white background

Question

Changing the text color to white when the background color is black works great using mix-blend-mode: difference. Move the mouse to the text to see the effect:

const blackBox = document.querySelector(".black-box");
window.addEventListener('mousemove', function(event) {
  blackBox.style.left = `${event.pageX - 50}px`;
  blackBox.style.top = `${event.pageY - 50}px`;
});

.wrapper {
  background-color: white;
}
h1 {
  position: relative;
  z-index: 2;
  color: white;
  mix-blend-mode: difference;
}

.black-box {
  width: 100px;
  height: 100px;
  position: absolute;
  z-index: 1;
  background-color: black;
}

<div class="wrapper">
  <h1>Lorem Ipsum</h1>
</div>
<div class="black-box"></div>

Answer 1

Here is an idea that rely on background coloration and not mix-blend-mode. The trick is to have a gradient with the same dimension as the image that you move the same way to simulate the blend mode:

const box = document.querySelector(".box");
const h1 = document.querySelector("h1");
window.addEventListener('mousemove', function(event) {
  box.style.left = `${event.pageX - 50}px`;
  box.style.top = `${event.pageY - 50}px`;
  
  h1.style.backgroundPosition = `${event.pageX - 50}px ${event.pageY - 50}px`;
});

.wrapper {
  background-color: white;
}
h1 {
  position: relative;
  z-index: 2;
  color: white;
  background: 
    /*gradient                   position   /    size  */
    linear-gradient(#fff,#fff) -100px -100px/100px 100px fixed no-repeat,
    #000;
  -webkit-background-clip: text;
  background-clip: text;
  -webkit-text-fill-color: transparent;
  color: transparent;
}

.box {
  width: 100px;
  height: 100px;
  position: absolute;
  z-index: 1;
  background-image: url("https://placekitten.com/100/100")
}

<div class="wrapper">
  <h1>Lorem Ipsum</h1>
</div>
<div class="box"></div>