@classmethod
def for_input(cls, value, validate=True):
    """
        Convert the fixed width into a CSS class.

        Args:
            value: the width.
            validate: check whether the width is a fixed value.

        Raises:
            ValueError: if validate is True and the value is invalid.

        Returns:
            str: the CSS class used to set the width of a Text input.

        """
    if validate and not cls.is_valid(value):
        raise ValueError('Unexpected fixed width', value)
    return 'govuk-input--width-%d' % value
