Norwegian Dagfinn R. Parnas has written a Master thesis on How to cope with incorrect HTML (it is also available as Postscript).
One of the more interesting facts of his thesis is this: He sampled a total of 2 500 000 documents from the Open Directory Project, and found that only 0.7 % of these documents were valid HTML.
The three most common errors was missing DTDs. Then followed specifying invalid attributes, and missing required attributes.