Unkown property: object-fit

Dear All! When I try to open a CSS file in the File Manager for editing, the editor has a warning that says: "Unkown property 'object-fit'. The website renders properly, so I don't really understand the reason for this message. I know that it's a small "problem", but it really bugs me... Do you have any idea what causes this? Is this a bug? Or a feature? Am I missing something? Thank you in advance! BR, Daniel