The original HTML documents recommended “be generous in what you accept”, and it has bedeviled us ever since because each browser accepts different superset of the specifications. It is the specifications that should be generous, not their interpretation. –Doug McIlroy, quoted from The Art of UNIX Programming, pg 21
This is the reason web development is so difficult. Every now and then I run across someone who considers web development to not be “real programming”. I suppose that is because web developers are not only faced with being generous in what they accept by way of input but also because they must account for a wide variety of environments (browsers), each of which requires slightly different output in order to achieve the same effect.