Answer 1

Because your are lucky ^^

Html is based on SGML, parent more permissive than it's other child XML: if a Html document isn't valid, the purpose of browsers is to display it anyway, as they can... Sometimes correction is relativly implicite, so browser right corrected, but you can be sure that with another browser the correction will be correct :P And often, auto corrections doesn't be accurate: in these case, most of document is still displayable ( an XML or XHTML document is less permissive, and will not display anything ). Search engines likes too find valid html, and risk to downrank in this case...