def _track_size(token):
    """Parse ``track-size``."""
    track_breadth = _track_breadth(token)
    if track_breadth:
        return track_breadth
    function = parse_function(token)
    if function:
        name, args = function
        if name == 'minmax':
            if len(args) == 2:
                inflexible_breadth = _inflexible_breadth(args[0])
                track_breadth = _track_breadth(args[1])
                if inflexible_breadth and track_breadth:
                    return ('minmax()', inflexible_breadth, track_breadth)
        elif name == 'fit-content':
            if len(args) == 1:
                length = get_length(args[0], negative=False, percentage=True)
                if length:
                    return ('fit-content()', length)
Parse ``track-size``.
weasyprint/css/validation/properties.py