The `<div>` element is not allowed to be a child of the `<button>` element. The `<button>` is an inline element, while `<div>` is a block element. This can cause rendering issues to occur.

# How do I fix this ?

Use the `<span>` element instead of the `<div>` element to customize the button.

# Resources

* [The div element](https://www.w3.org/TR/html5/grouping-content.html#the-div-element)
* [The button element](https://www.w3.org/TR/html5/forms.html#the-button-element)