Open DevTools Inspector and in the Rules side panel add background-size property. Then try type c and wait for cover or contain to be autocompleted, nothing will happen. If you type i, initial will be offered correctly, only the keyword values are missing. If you press down arrow, a menu will appear, again missing the keywords. List of available values on https://developer.mozilla.org/en-US/docs/Web/CSS/background-size
I think this one is a dup.