[danudey]

The Robustness Principle states "Be conservative in what you do, be liberal in what you accept from others."

Following that maxim, browser developers assumed that, even if HTML wasn't inherently correct, if they could figure out what the user logically meant then assuming that was better than not working at all.

In short, people wrote garbage HTML and it proved easier to fix browsers than people. At first, it wasn't too problematic, but as HTML got more complex more problems surfaced and now everything is a mess.

This was the goal of XHTML: HTML that was required to validate as XML or it wouldn't work at all, and some browsers were, indeed, strict at this. The idea was that you'd only use XHTML if you were generating it with an XML parser or some other template generator that could produce valid code. In reality, that just meant that browsers that didn't understand XHTML treated it like HTML and worked, and browsers that did understand XHTML and validated it would show errors. Thus, users saw that browser X (doing the right thing) couldn't display a site, but browser Y (doing the wrong thing) could.