The general principle of robustness according to Jon Postel is as follows:
Be conservative in what you do, be liberal in what you accept from others.
It is a very captivating statement and I leave it up to you to the see the boundaries of its essence.
The quote is available in RFC 761 as follows:
2.10. Robustness Principle
TCP implementations should follow a general principle of robustness: be conservative in what you do, be liberal in what you accept from others."