Show alert if style is not supported, without HTML

I would need something like this:

if {
  @supports not(display: grid) {
    window.alert("Please use a different browser.");
  }
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

So if CSS grid is not supported by a browser, it should show a JavaScript alert. I know how it would work with just CSS, but I need a JavaScript or jQuery solution, without any HTML in the HTML section. How is it possible to do that?

>Solution :

You’re making this too complicated 😀

if (document.documentElement.style.grid === undefined) {
    alert("Please use a different browser.");
}

In modern browsers, grid is a property of the style of every element. If it’s undefined, then the browser doesn’t support it.