The fit-content keyword is equivalent to fit-content(stretch). In practice, this means that the box will use the available space, but never more than max-content.
When used as laid out box size for width, height, min-width, min-height, max-width and max-height the maximum and minimum sizes refer to the content size.
The interpolate-size property and calc-size() function can be used to enable animations to and from fit-content.
Note: The CSS Sizing specification also defines the fit-content() function. This page details the keyword.