Principal flow CSS

In order of creating my website, I'm getting following warning in Firefox broweser:

When rendering the element, the used values of CSS properties “writing-mode”, “direction”, and “text-orientation” on the element are taken from the computed values of the element, not from the element’s own values. Consider setting these properties on the :root CSS pseudo-class. For more information see “The Principal Writing Mode” in https://www.w3.org/TR/css-writing-modes-3/#principal-flow

I don't know what's wrong with it and what should I do.

Is there anyone who can figure this out?

I have already tried

:root{
    direction:initial;
    text-orientation:initial;
    writing-mode:initial;
}

body,html,:root{
    direction:initial;
    text-orientation:initial;
    writing-mode:initial;
}

But still the warning is there.

Im using material-ui BTW.