def _compute_track_breadth(style, name, value):
    """Compute track breadth."""
    if value in ('auto', 'min-content', 'max-content'):
        return value
    elif isinstance(value, Dimension):
        if value.unit == 'fr':
            return value
        else:
            return length(style, name, value)
Compute track breadth.
weasyprint/css/computed_values.py